Fred,
First of all I'm glad you are deciding to give turkey hunting a try, it's a blast. As for the gun there are a lot of choices and options. As for gauge, either a 20 or 12 gauge will do. 90 percent of people probably use the 12 gauge but more and more people are switching to the 20. Although it doesn't pack quite the punch of a 12 gauge a 20 gauge with the right load can kill them cleanly out to 35 yards, and some of the new load and chokes claim you can stretch that even farther.

You should use loads that are made specifically for turkey hunting in shot size 4,5,or 6. I personally use winchester supremes in #6s. As for length of barrel I prefer a nice short barrel. It is easier to handle and carry around, I think my turkey gun has a nice short 21" barrel. As for a choke tube, a regular factory full choke will work just fine, but most people opt to use super full chokes and there are tons of good ones out there. My 870 has the Remington Turkey choke that came with it and it throws pretty nasty patterns. Camo is a nice, but expensive touch, and it isn't really necessary. many turkey have died to plain old blued steel and wood stocks, just make sure the metal isn't real shiney.

Turkey loads and chokes shoot such tight patterns now, most people have gone away from just using a bead to aim and have gone to rifled sights, red dot sights, and scopes. I personally like a low powered scope, mine is 1.5-4x with the turkey plex reticle. That way you can be really precise when you take aim on those 20 yard shots and your pattern isn't much bigger then a softball. The action of your shotgun is a personal preference, I shoot an 870 because they are fairly inexpensive and I love them, some people prefer a semi auto because turkey loads have notoriously hard recoil and the semi soaks some of it up. Anways thats a lengthy answer to your questions, I hope it helps. Feel free if you have any more, and check out the turkey hunting section if you have any hunting questions. good luck!