I've been writing songs since I was a little girl. I understood that, generally, there was a verse, chorus, and a bridge. Of course, my songs back then were a little...childlike and reminded me of Avril Lavigne and stuff.
Now, though, I write songs that are more meaningful to myself. And then some songs I write just for fun!
The key for me to write something is just come up with some chords or a good riff, then I just start trying to come up with random lyrics. A lot of the times, I don't know what the song is going to be about, until I find a line that I really like and stick to it.
What I also do when I don't know how the lyrics are going to sound with the music, I just kind of play the music on the guitar and hum along with it to come up with a good tune, then make up some lyrics.
If all else fails, I record the music on my phone for a good minute or two, then play it back and work from there, humming, or singing random lyrics. It helps because I'm not focused on playing anything while I focus on writing lyrics.
It's a process, and it takes a while to write a song that you are 100% happy with. You'll find yourself revising your lyrics or coming up with a little something to add to a song even after you think you've finished it. Rarely does it happen where I write something in a day, and am totally happy with it.
As finishing touches, once I'm pretty happy with the song, I go back and play/sing it a few times and add where I would like to spice it up. For example, the strumming on the guitar becomes more powerful, or I raise my voice so the words are more powerful.
It can be a very fun and difficult process sometimes.
Just have fun with it! (: