For starters, you'd need to purchase one-year App Hub subscription ( [ Register or Signin to view external links. ] ) and obtain Visual Studio 2010, or [ Register or Signin to view external links. ] along with [ Register or Signin to view external links. ] .

The membership is tied with your Xbox LIVE account, and is required to deploy/debug on the console. On other platforms (or for unmanaged development on the Xbox) you'll need to get yourself a developers license for that platform (which is the tricky part since you need to prove that you've got what it takes to develop for their platform and need to actually establish a reputable company), sign some NDA's, drop a bunch of dough, get some dev kits, and then start the development process.

Indie games on Xbox is definitely the way to go.

Also on top of what -CLK- said you can check this topic here made by our admin SPEED of how to legally obtain this membership for free. Please note that doing it this way that you cannot submit games to the marketplace and you cannot review other peoples games. For that you do need to purchase the 100 USD membership, which is well worth it in my opinion.