If you are drawn to Mastiffs, the Pit Terriers and all of their various combinations then you have to ask yourself serious questions about all the variables in your lifestyle before getting one. You said you've never had a dog before, that's huge. Other things are your house/yard situation, family members (kids), frequency of visitors, your personality, the way you interact with love ones, your time available to train the dog, how social will you be with your dog like going out in public, local laws/HOA stips/Apartment condo rules etc... There are a lot of things to consider.
Once you figure that out, then you have to get a dog from a reliable source. The biggest mistake people make is getting a dog that's game. I can hook you up with a 100% professional, Biff & Buffy, AmStaff breeder and you still might get a dog that's a handful and getting a roller when you're not prepared for it is no bueno.
I grew up raising aggressive guard dogs. Terriers can be a crap shoot. My sister has the greatest gun metal blue AmStaff ever. Looks like Satan's spawn but cuddles with kittens. When I move back to AZ, I'm gonna start breeding Hog Dogs and he's going into service.
All i say is be careful in every aspect of the process.
If you like Asian things, get one of these: