What kind of playing style do you like? Do you like lots of shooting? Do you like close combat? Do you like tactical flexibility? Do you like toughness and the ability to take and dish damage despite lack of considerable numbers? Do you like a REAL MAN'S ARMY?
Well, if you picked the last one and the first one, you want to be recruited into Our Holy Emperor's Blessed Imperial Guard.
Otherwise, Tau are a good army, but not easy...
A forgiving army would be the Space Marines (AKA: SMurfs), but the over abundance of players that play SMurfs have tarnished the name *stomps on it* I would suggest you avoid that army (Or any sub-army of it, such as the Blood Angels) unless you absolutely have to.
Once you make your choice, pick up the Codex for that army, and if you have the money on hand, the $100 (roughly) box army to get started. If you don't have a friend with one, get a Rulebook, as you need to know the rules to play.
While you are collecting, keep this fact in mind: This is a HOBBY, NOT just a game! It will be expensive. If you don't have the income to maintain that, I suggest you rethink your idea of starting this hobby.