The first thing you will need is a passport valid for 6 months beyond your planned travel date. Visas will not be issued otherwise. If you are landing in Nairobi, Kenya they now require a visa (transit visa for $20 which can be purchased on arrival and departure) and you can acquire a visa for entry into Tanzania at the border or when you land. The current fee is $50 US, correct change is useful. If you want to acquire your visa prior to travel, you can reach the Embassy of the United Republic of Tanzania at:

http://www.tanzania.go.tz/immigrationf.html