rep ++ to the digital pimp - excellent post me lad.
The best advice I can give is:
a. Just keep practicing. Write and write, and though you will have tons of crap songs, hopefully a couple will be half-decent. But writing is the only way to improve... your writing.
b. Take constructive criticism well. If 100 people tell you that line A sucks, you should probably consider altering it. But also remember that as long as you are happy with the song, that is all that matters
.