Looks like a BI (Boa imperator) to me! True red tails (Boa constrictor; BC) are a bit harder to come by. BI are sometimes referred to as "red tailed boas," even though this designation truly belongs to BC. This is probably the case with your girl. The fact that your boa's tail is brown is also a good indicator that she's a BI - most BIs' tails fade to brown as they age, while BCs' stay a brilliant red.

BIs are much hardier than BCs, so they make better first boas in my opinion. The "Boa FAQ" sticky in this sub-forum has some useful information. Good luck, and don't be afraid to keep asking questions!