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