Majority of the BM players go fire build. However, lighting build is better in PVP. Go whichever style you like. (If you have vt badge, fire BM has cool animations)
Also, being BM, no matter which build you go, you need to learn how to tank. A BM who doesn't know how to tank boss is pretty much garbage. Also, 9 out of 10 times, if you are in a dungeon party, people expect you to tank. If you have decent gear, but can't hold aggro on boss, people will start teasing you and making fun of you. This is how toxic this game is.