A friend of mine once told me a story about how a fellow student in his songwriting class at Berklee could only play six songs. My friend at the time could play a couple thousand (when you know Bruce Springsteen's and the Beatles' entire catalogue, songs add up fast).
How do you learn how to write songs? By learning how to play songs. That's where you start. At least that's where I started along with many of my friends.
If nothing else, January provides a great place to start developing routines. Learning more songs is a routine I'm getting back into. Songs beget songs, and what songwriter doesn't want more songs?
It really doesn't take much. If you're a beginner, it's great practice for ear training (I'm not against tabs, but I encourage using your ears as much as possible) and building your repertoire. If you're not a beginner, it's still great for ear training and building your repertoire. Find the key, find the chords, learn the structure, go from there.
I'm off to do just that.