Mael

yep, you got it write (write, get it? haha... i made a funny...)

anyways, on topic, i'd recommend you look into some songwriting books. while they can't teach you what to say or how to say it, or how to write like a natural, they can help with explaining how songs work, why they work, things to practice in writing, different styles, etc. basically, they can give you starting point for practice of lyric writing, kinda like how a book on how to draw doesn't teach you what to draw, but techniques and stuff.

also, if you don't have them already, buy a thesarus and rhyming dictionary. and as someone said above, keep a tape recorder around at all times. or at least a pad of paper and pen.