You have to beat the Pokemon League to get him, sorry.
Once you beat the game you go to N's castle and talk to him and walk around for a bit. Then you battle his Reshiram which is lv. 70. Then he gives the White Stone, which is Reshirams dormant form. Then go to the top of Dragonspiral tower and awaken it. You catch it at lv. 70. Utra balls and quick balls work well, or use the Masterball if haven't used it yet. Good luck!