How can i build up my neck?

Whey protein and creatine'll do it.

I own a pit bull and can't imagine why you would want to make the neck muscles any thicker and strong than they already are. Is fighting your dog in your plans (which is illegal). My advise forget the muscles and enjoy your dog. These dogs are strong enough as it is (mine almost pulls me down when walking her) without making them any stronger or more aggressive.

Tackle by leading with your head.

Shrug exercises. dumbells in your hand at your sides and raise your shoulders in a "shrugging motion".....; it works. Look it up to see how to do it.

give head... my gf says her neck is sore afterwards

Stare - Oids... get it