I am trying to record a demo for my band, but im not sure how to record the drums.

I have mics already.

Will I need an interface? mixer? anything else and how will I set it all up?

I used my Digitech RP250 to record guitars and bass on my MacBook with Garageband and i think is sounded good.

Anything will help. Im just a little puzzled and cant find what im looking for on the 'net.
hmmmmmm, well you need to plug your mics into a mixer. i think that is what you do and then plug the mixer to your macbook then garageband.

try googling "recording studio drums" something along the lines of that.

if not try going to a local recording studio and asking the guy who owns it, or try a music store
IMO The best way to go is with an 8 input interface such as the PreSonus FP10
The firewire connection means you wont be mixing down like a standard mixer or USB interface will. This is great for tweaking drums after you record them.

The FP10 runs around $400 - $450 these days which is a nice price for such a great interface.