First off, it's been recommended by some that you start out with a percussion system rather than a flint system if you're starting on muzzleloaders as they're easier to work with.
I have no experience with the muzzleloaders of Thompson Center or any other company, but I've heard good thing about Hawken rifles in general so I don't think you can go wrong with them.
As for what you'll need, you'll need a powder measurer, nipple wrench, ball starter, lubed patches for shooting round balls, and black powder solvent. These can be obtained at MidwayUSA, Cabela's, and several other sites that deal in black powder.