I want to record my voice guitar and bass and drums.. I have the instruments what do I need to feed them into the computer... I also have a presonus interface audio unit . But will that work with garageband? Basically I am asking what I need to record Beatles tunes .I can play the guitar, bass and sing but I will probably get a drummer to help me

Assuming you plan to record the parts in one by one it sounds like you have everything you need. If you haven't worked in GarageBand before, a basic course should be able to get you started—I've linked to one (above); the video below (from the course) covers hooking up and setting up the interface..