I remember I once saw a movie that remained inside my mind as they were saying that it was based on a true story and now I really want to find more about it but sadly I forgot it's name.
I remember that after graduation, a guy(named 'Alex'?) was finally free and decided that he doesn't want this kind of life anymore and become a 'wanderer' through USA.His sister was the only one that knew about this, while his parents were very concerned about what could have happened to him.
He was just randomly traveling through the land and I remember once he arrived to a trailer camp where a under-age girl tried to convince him to make love with her, but he didn't.
In the end, he died of some kind of food poisoning in a abandoned car.

I would really apreciate if you can help me find the title of the movie ;)

That sounds like Into the Wild: Into the Wild (2007) - Synopsis (http://www.imdb.com/title/tt0758758/synopsis?ref_=tt_stry_pl)

Cat is right...it was actually a pretty good film and an interesting book.