Will has covered all that you need. You should have Garageband on the Mac which covers the
recording software. There are loads of low priced audio interfaces, the one Will has
suggested is good. You can have a look around, but the simple requirement is that it has a
mic pre-amp built in and if you are using a mic that requires phantom power, that it has
phantom power as an option. You could get a dynamic mic such as an SM58 (phantom power is
not needed), which can be used in the hand or with a mic stand and will record adequately
enough (and last forever!). Steer away from ebay as there are lots of copies out there.