Pablo,
I think this is a typical problem. IF I get a line, I'll write it down but any more, I want to map out my song first. I equate that to entering "Best Buy"(big box electronics store for those not in the U.S.) looking for the car radio section. Normally, I know where I'm going in that store so I just start walking. But I didn't this time, so I just stood inside the front door, glancing at the signs overhead until I saw "car audio".
That's the way I like to write now, first getting the big idea first. Then, I like to map out the idea such as verse 1, girl finds out boyfriend is cheating on him and decides not to take it anymore, verse 2 she decides to put his testicles in a vice(ouch!!!!!!) while he's asleep, in the bridge she also decides to brand the word "cheater" on his butt.
Okay, stupid examples but that's the way I like to do it. I find I generally(at least hopefully) know what I want to say in the chorus so its the verses and bridge where I need to map the song out.
Also, before writing, I like to brainstorm, my favorite technique is either writing down any idea that comes to mind or "clustering" which is a form of brainstorming. I equate it to gassing up the car before I start driving on a long trip. I can always do more research later. Hope this helps.
PaulB