welcome to the forum mate.
basically, the best thing you can do is go to a boxing gym, and learn the real thing. at the monment youll just be playing patta-cake with the bag, and itll stay that way untill you learn how to do it properly.

i suggest that cause of your size you get the biggest bag you possible can. DONT GET A WAVE MASTER. we used to have one at the gym and every time somone threw a punch at it fell over. they also skid all over the floor when you hit them, which really is no good.

as far as it goes with bags, get the most protection you can when you hit it at the moment, you wont know how to hold your hands, and you could do damage to your wrists