Unless the cable that connects the guitar to the Mac has some additional electronics included, electric guitar piped into a Mac mic jack is gonna sound cr@ptacular.
You need to think about the things you want to record, and the purposes, and your budget.
Then I recommend that you study up on some of the basics with books and magazines before making a decision. Hit the library. Good magazines to read are: Sound on Sound, Recording, Electronic Musician, Keyboard, Future Music, Computer Music, Mix. They often have articles on how to record acoustic and electric instruments, and microphone selection.