1. Listen to 1 new artist each week
Probably one of the most important things I can recommend is that you are constantly seeking new artists to listen to. Find a new artist (or 2 or 3 if they only have an album or 2 out) and spend some time with their catalogue over the course of a week. It's really important to appreciate and understand a wide variety of music when you want to write so you don't just wind up sounding like your favorite artist *Cough*GretaVanFleet*Cough*. To avoid sounding stagnant or "done" absorbing tonnes of new music can be a great way to get your head out of the box musically. Even if your not really keen on the artist just run their catalogue through a few times on Spotify. It might just grow on you. According to the NCBI, we actually wind up liking music the more we listen to it. Sticking to this routine should expand your musical mind and turn you into a songwriting guru!
2. Learn your favorite songs on each album
Between your listening sessions be sure to remember to pick up your instrument and try learn your favorite songs in their catalogue. I know a lot of users like to use tabs but I highly recommend training your ear by just trying to learn the song without tabs. This will be really hard to a beginner but over time you will really appreciate having this skill. I've been learning by ear for over 10 years now and it gets quicker the more you do it. When it comes to jamming with other musicians this is invaluable. Back to the point though learning all these new songs from your artists you discover will help absorb new skills and tricks into your playing. When you go to jam you'll find that you mix and match tricks from different genres and it will make your playing more fun and interesting to listen to.
3. Analyse the lyrics
Songs aren't just about the music! Great lyrics are often key to a song. What you'll notice with a lot of great artists is that their lyrics nearly are derived from the music and the topics they choose to speak about, are a reaction to the atmosphere that the music creates. A sad piano intro will often be accompanied by a sad story lyrically. Obviously there are exceptions to this but study those exceptions and analyse how they are making that interesting. Lyrics are essentially poetry and often tell a story with a start, middle and end. Study this closely and when writing your own songs try to replicate these structures.
4. Get outside your comfort zone, explore new genres!
Similar to the new artists discussion above, it's super important to ensure that you aren't just listening to a bunch of artists in the same genre. Growing up as a teenager, I exclusively listened to rock and metal. As I've aged though I've picked up genres like hip hop, pop, funk, jazz, classical, reggae, electronic, dub step. I find when I write music no I'll often mix some of these different genres back into rock and I find it makes it a lot more interesting.
5. Learn 1 new scale each month
Probably a less fun option at first but more valuable if practiced properly is taking on new scales. Scales are homework-y and boring but learning a bunch and really understanding when to use each will really make you stronger at writing compelling melodies and solos in your music. The best way that I've found to learn new scales is to open up this bad boy here and just pick any new scale and spend the evening with it. Be robotic at first and just go up and down the patterns until they sink in.
6. Spend an evening each week improvising solos on top of your new scale
After you can remember the scale without looking, go to youtube and search for a backing track for that scale and spend another evening trying to improvise over the track. You'll find that you can't make it sound right for awhile which is totally normal. Keep spending time with it though. You'll find over the course of a few evenings you might have a few licks that sound good over the backing track. Bonus points, try to compose a solo with the scale with a start middle and end. Again spending a lot of time with 1 scale is super important to truly absorb it and all it's nuances, that's why I recommend spending a month with one scale. Another trick for absorbing the scale is googling songs that use the scale you are studying. Learning those songs will also highlight interesting ways in which you can use the scale.
7. Learn other instruments
Understandably this tip might not be for everyone. Not everyone can afford to just grab a new instrument which is fine. However if budget allows and you already play guitar a really easy win is buying a cheap bass. Following all the tips above and learning the parts on your new instrument is absolutely invaluable. If you know how multiple parts of songs should go together this will level up your song writing ten fold. Over the course of my musical career I've picked up all the standard rock band instruments(guitar, bass, drums) and have a pretty decent level at all of them. Knowing how to apply all these instruments in a song will really help when you're composing your own music!
8. Try write something on another instrument
If you're like me there's only so much you can get out of one instrument and you can just get stuck sometimes. You might have written no new guitar riffs in awhile and everything your exploring sounds done. I find that picking up the bass or even the drums can be a really good way of starting something new with a song. Finding a drum beat you really like and then building on top of that can lead you down a new path that you wouldn't have explored on the guitar.
9. Spend time just jamming with other musicians
This one is critical. Obviously it's great to be doing a bunch of study at home but actually playing with other humans will show you your flaws and highlight areas you need to improve. Players often struggle with rhythm when playing with other people if they're normally not used to it. Make sure to really pay attention and use each others body language to follow each other. You'll also pick up tricks from others. They might do a lick or a new chord you've not heard and they might have recommendations for new artists to listen to. If you're in a city try to get out busking as it's a sure fire way to meet new musicians. More isolated areas could avail of things like Craigslist.
10. Study your favorite bands songs
We discussed exploring new music earlier which is super important but don't forget where you came from. Whoever your favorite artist is, make sure to learn as many of their songs as you can on as many instruments as you can. Make sure to also ask yourself why you like them so much. What elements of their music are your favorite? What scales do they rely on? What types of rhythms? How can you apply that to your music?
11. Try to hone a style for your band
So you've listened to a bunch of new music and learned tonnes of new scales and songs. From doing that alone you will have gained an idea of what your favorite style of music is and what elements you want to bring to your own music. Ask yourself what is the theme of your band? What is the primary mood/atmosphere of the music you want to create? What other bands have done that before? How will you be different? What elements from other music can you mix to make it more exotic?
12. Make your songs coherent
When I started writing my first album I feel into a bit of a trap. Because I listened to so much different types of music I was finding that listening to my songs back to back didn't make a huge amount of sense. One song was a cheesy rock ballad, something you'd expect from the Foo Fighters. Then the next was a swing/rock kind of song, think Guns N Roses meets Imelda may with a dark twist. This obviously didn't work and we wound up scraping the Foo Fighters song, even though it was a good song, in favour of the more unique atmosphere we'd created on the other song. We tried to stick to that style after wards. It can be tricky sticking to a style but ensuring that all the songs sound different to each other. Changing things like speed, rhythm, and key is an easy way to not sound samey. Be sure to have some key ingredients in your music though that you can use to glue them together. Also don't worry about making your music too different either, look at AC/DC, they wrote the same song for 40 years and did just fine. Just make sure it's a damn good song like they did if you go that route.
13. Ask what you want to communicate with the song
I like to think of music as a snapshot of emotion. The best songs evoke powerful emotions from us. To do that everything must line up. All the instruments have to use scales that serve the emotion you are trying to convey. The percussive instruments largely control the amount of energy. Use them to bring a song up and down but use the musical instruments to control the mood. The lyrics and singing should also match this mood. If the song is blistering metal there should be some high energy vocals in there. Mumble rap probably wouldn't work to well with it. Although try it ya never know it could work!
14. Pick a random mood and try to write a song for that mood
As an off shoot to the previous point try and exercise your control on emotion. Choose a mood once a month and try to write a song for that mood. A good exercise would be to try exaggerate that mood as much as possible. Try write the happiest song you possibly can, or the saddest. Doing this will put you more in control when it comes time to make a song for real about something in your life. You will be able to create a mood and atmosphere that matches what you are feeling.
15. Learn how to record yourself
In the modern day it's becoming more and more common to self record in a home studio. You can get recording with as little as a guitar, an audio interface(plugs your instrument into pc), a DAW(audio recording software) and a laptop. You don't even need an amp. You can get digital ones. I personally recommend Helix Native. Recording yourself and laying your tracks down so you can actually hear them will help a lot in determining what the song needs. Ideally you will be able to play a few instruments and you can just start knocking out full songs yourself. But if not invite over your friends and get recording. Hearing yourselves will highlight what you need to work on.