I don't really agree with people who say study theory as the be all end all of songwriting. It can help but it isn't necessary.

Things I do, as a unsuccessful but semi prolific songwriter (your milage may vary of course):

1.Record all of your ideas, I have an app on my phone (evernote) that does audio as well as text, I record every coolish riff I find when I am noodling and I verbally explain the riff to myself so that if I hear it a year later I can figure it out. Also If I hear or see anything interesting I toss it in my notes for inspiration later. I have lots of lyric fragments that I can crib from because of this.

-Don't be afraid to rip off what you like. If you love Black Sabbath try writing riffs that sound like Black Sabbath. If you hear something youhelp, try to play it without looking at a tab, its kinda like when doctor Seuss would try to draw animals at the zoo and developed his own style. You too will develop your own voice eventually and to me, however counter intuitive this sounds, it helps.

-It's important to try, a lot. You will write some stinkers, not everything is going to be great, but the more you practice songwriting the better you get. Try different genres. Try writing a punk song or a country song or a whatever song, get your head in a different mindset. You aren't going to use every song, and if you are like me you will only use like one out of every ten, but its practice and it helps. And who knows, you can always rework and find inspiration from a stinker.

It is also good to have a friend who can be critical to you about your songs. But also you don't always have to agree with them.

Hope this helps at least a little.

Tl/Dr practice mostly.