If I were new to guns the first thing I would do is purchase the book Cartridges of the World. Start reading there and then go shoot some rifles of different calibers to see what you like. Also, ask around in the Guns or Firearms forum they will be a lot of help. Not that it's a bad question but it's more appropriate there. Actually if a Mod could move the post to there you'd find better results.
Anywho, start with that book. It's a little pricey $35 I think but well worth every penny.