First off you will need an audio interface, there are 2 kinds external and internal (Pci Card). Google audio interface for references. Some are better the others, external or internal doesnt matter as much. This is how you plug your guitar or any instrument into your PC.
I use external because its easyer to get to the inputs ( shhh …) next you will need a DAW, that means Digital audio workstation. Like Cakewalk or Protools, expensive but there are some for free like Rapture or audaicity (yuk) for recording, mixing, mastering. I might also suggest buying the book “Home Recording for Dummies”, lots of great techniques explained simply. As for the toon track drum kits and other stuff, you need to listen to the demos and figure out what your gonna want or need. Of course Metal foundary is a great place to start!
Here are some considerations I think you should think about. You mention that your a guitar player, and you just wanna toss some riffs down for your bandmates. Dont forget you will need to purchase superior drummer first and then get the MF sdx addon. this is a hell of a purchase for someone just wanting to throw some riffs down. In the long run you will have a the best of the best and maybe your drummer will invest ina pad kit and you guys and use MF sdx for great sounding recordings and dare I say ona laptop can be used for live shows =D. In the short term, consider buying EZ drummer with Metal machine and Drum kit from hell addons and then get the midi paks for monster and/or extreme. this will allow you to spend less time worring about building a kit and more time picking drum beats and fills for your riffs.