There are a number of odds calculators out there. When I first started playing, I bought something called Poker-Pal. Basically, it calculates the chances of hitting your outs and also purports to give you odds on whether your hand will win or not. I used it for a very short time, but would definitely not recommend that (or anything similar) for several reasons.
1) It could make you lazy, and that can never be a good thing. It's easy enough to work it out for yourself, so get into the habit at the outset
of working it out yourself. The easiest way for a novice is to count your outs, then double it. The result will be your odds of hitting one of your outs. So, if you have 2 overcards that's 6 outs. Your chances of hitting one of them on the next street is therefore approximate 12%.
2) These programs are not very clever, and will often mislead you. Lets say you have AJo and the flop comes 89T. The odds calculator will show that you have 12% chance. If you were getting pot odds
of 10:1 you may be tempted to play the hand. Probably not a good idea, because if your J comes on the turn, you make well be drawing dead to a straight. What looks like 3 outs to a machine, doesn't (or shouldn't) look like 3 outs to a human!
3) The odds calculator takes no account of who you are playing against. That is often as important (if not more so) that what
you are playing.
4) Many of these devices are banned by some sites. Before you buy, check with the site(s) that you are likely to play at.
AG has already made a very strong case for Poker Tracker and Gladders has mentioned Poker Office. Having used PT for quite a long time myself, I would definitely recommend it, it's been invaluable to me and it's also cheaper than PO. The new version of PT (due for release in January) looks even better. I would also add that PT support is outstandingly good if you need it, and there are many people who use this forum who also use PT and would be able to give advice. There is also the sticky that Chuck has prepared giving information and advice on PT's use.