It is almost certainly out of your reach at present but the finest and most widely acclaimed maths' software on the market is Stephen Wolfram's Mathematica5.
Out-of-reach because the list price is around £2k. If you are at college you may have access to it through a university web site. Some USA campuses let you browse, though scope is limited.
What is also remarkable is the manual for Mathematica5. It is hardback, nearly 1500pp, and a good buy as a source of maths/statistics formulae for any student, AO level to postgraduate. This book was on offer under £30 from Amazon UK and I suggest you press your school or college to get a copy. Don't be palmed off with the earlier Mathematica4.