The opponent you are playing against chose for him/her to play as White and for you to play as Black. It might have been set for random and you just happened to end up with the Black peices. Hope that helped mate!
if chess.com starts you off as black, you have to work your way up in order to play white. here at chess.com we believe that "first-movers" (aka, those who "play white [and run]") are required to merit the privilege to go first in a given game.
I have played/playing two games on Chess.com so far. Both times I have been the Black pieces. How do you play white?