This has been already answered in the manga, but maybe they haven't got around to it in the anime version yet. I'll tell you but only if you don't mind spoilers.
Luffy and Ace aren't actually brothers by blood. Ace is the son of Gol D. Roger and a woman with a surname Rogue D. Portgas. Portgas (his mother) carried Ace in her stomach longer than you are supposed to carry a baby in order to protect him from the navy who were looking for Roger's son. As a result, she died during his childbirth and he took up her last name to honor her sacrifice. (Also cause he hates Gol, his real father) Since Roger and Garp had a close bond, Roger asked Garp to adopt Ace while he was in jail. Garp did so and hence Luffy and Ace became brothers.
There is also other stuff involved, but keep watching cause it gets epically exciting!
Check out the One Piece wiki to spoil yourself more http://onepiece.wikia.com/wiki/Main_Page
But I recommend to keep reading the manga or watching the anime instead cause its much more fun that way! =D