If you have papers, you can compete without much issue. If you dont, you can do a limited registration through the UKC. They require that your dog is spayed/nuetered. This allows you to compete in all UKC sports except for comformation. You shouldn't start weight pulling your dog until he's ATLEAST 12 months of age. This is because when he's younger his bones are still growing and you can cause serious joint and bone issues that will effect him later on. The ideal age is around 15-18 months.
You can either get a training weight pull harness to start, or get a competition harness. A good harness will run you 50+. Google Table Rock Harnesses. Good stuff. When you start with him it's just a matter of teaching him that it's FUN to WP. You start off by putting the harness on him and just having him walk around, getting used to the idea of the harness. You'll do this several times until he gets used to it. Then you can start with a jug full of rocks, and just walk him around the yard pulling it behind him, to get him used to the idea of having something behind him. Then you can slowly add weight. I've seen people use everything from tires, to logs as training tools. When he gets comfortable with pulling, then you can get a sled, and slowly work at adding weight. A dog will pull the weight over a 16 foot length. Don't over work him, he will not quit, he's a pit, quit isn't in his vocab. You have to watch him and know when he's had enough. Good luck!