Hey Gilly, welcome.
Don't worry, you're not alone. So many choices out there it gets confusing. Here's a pointer for you that may help. As far as when you get your Mac, I read everyone loves GarageBand.
Keyboards, well you've just got to look through http://forums.songstuff.com/forum/53-keyboards-synths-and-samplers/
I'm doing a blog about going through the motions of installing and figuring out how to create stuff on a pc so you might want to check in to my blog as I update it.
My post above got great responses from the guys here who can use the pc studio progs, so dig around the site as there's a wealth of advice here for someone like you and me.
Let me know how you get on with the keyboard buying (I'm thinking of getting something to try and teach myself to play). Any help you need, just holler.