I've been playing writing songs ever since I started playing guitar but it was only recently they became decent and so I started thinking why:
My guidelines are simple and I hope they help some one.
1: Write about what you know.
If you like black mass and killing children don't try writing a song about cutsey sheep and Jesus (and vice versa)
1.5: Or not...
Ignore step 1 if what you know is boring, ie if your only passion is music and stamp collecting don't write about stamps, no one will care! (no offense UG stamp collectors)
2: Don't over think.
Don't sit there thinking about what the song must sound like and what load of theory bollocks it relates to. Just get your guitar find a good tone and play until you find something that makes you go "Wow!" then work out key and what else you can do with it.
3: Make notes.
Get a note book (I'd advise a A4 one over A5 or smaller) and every time a bit of lyric or a bit of song come to you and write it down and when your feeling it come back to these skeletons of songs and try to finish. However, do not force your self it will only result in a low quality song. Try to get in your mindset from when you started the song to keep the song uniform through out.
4: Variety is the spice of life.
Don't make all your songs the same, people will get bored (unless of course, you are; A: part of a legendary rock band of 20+ years or B: The Ramones (which I doubt)). By all means have your own sound but all your songs should stand on there own individual songs, if many sound the same alter some a little bit to make them individual.
5: Be your self.
Many young song writers (like me until recently)write songs that are obvious steals of the last song they listened to. However it can be good for inspiration to base your songs on songs you like, but disguise it, as David Bowie said 'Talent borrows, genius steals" but he's never been done for plagiarism because he hides it.
Thanks for reading and I hope this helps.