with all due respect, let's not go into mastering, it's a whole different ball game which you probably won't be needing any time soon, so don't worry about that. protools you probably also don't need, while protools handles audio well (imo, best of all DAWs), it's midi sequencing is 100% shit, useless, garbage. you also need specific hardware to run protools.
i'd say garageband would be something to checkout (if you're running Mac that is), it's focussed mainly on pop music.
if you're on windows, i'd suggest you get yourself a good DAW (digital audio workstation) for ex. Cubase, Nuendo, or apparantly YC's favourite, Sonar and with that DAW get yourself the new Native Instruments Kontakt 3 (
NATIVE INSTRUMENTS : Products : Sampling Line : Kontakt 3) which includes a shitload of instruments for you to play with.
oh, also get a midi keystation, these things go for about a 100 bucks, be sure to get an m-audio one, they rock.