I use fruity loops with a midi keyboard and a bunch of sample libraries. Lots of people use CubaseSX, Reason, Cakewalk, and GIGAstudio
They've all got their strengths and weaknesses... fruity loops is considered the "beginners program", but I've been working with it for a few months and it's very powerful.
I'd suggest PMing Noct, Edgen or Chris Hurn if you want to talk about composing soundtracks
It depends on what skills you have musically. If you can read/write music etc. then you will want to look at programs like Cakewalk, Reason etc. as dj200423 suggested.
Programs like GarageBand are nice for those who write their own stuff, but also those who want to use loops/pre-written music. You may want to check this out.
Fruity loops is also a nice program to use. I don't use it, but I have checked it out in the past and seems like a nice environment to work with if you don't need a notation view. (You use keyboards, midi piano roll etc.)
Remember there are a number of composers at the board now (check the Hero-Fest section), and it shouldn't be a problem getting a composer for your project.
Perhaps you can give us a bit more information on what you are trying to achieve. For example, are you looking to write original music, or just mix pre written stuff together?