Audio interface or mixer to record guitar for beginner?

So I've been playing guitar for a while now and i want to start recording. I don't know much about it so i need some help. I have a MacBook Pro and garage band is what I'm using. If i just want to do some simple guitar and eventually vocals, and i have a keyboard that i may want to use to record, should i use an audio interface like M-Audio fast track or a mixer like Behringer Xenyx 802? What do you think would be the best to use with plugging in a guitar and mic? And i don't want to spend a lot of money on stuff.