First, start with GarageBand as your recording studio software; it came free with your MBP, and is a good "beginner" audio workstation app.
In order to record vocals or acoustic guitars, you will need a microphone of some sort. filmweaver's recommendation is good for live/"field" recording. For the studio, you may want something that can capture more nuance. I recommend Audio Technica's AT2020 USB or a Blue Snowball; both connect via USB so that you don't have to have a separate mixer.
You could use GarageBand's built-in virtual keyboard for constructing music and beats, but you'll tire of it quickly. For better hands-on beat construction, the Akai MPC-26 will do the trick. For playing musical passages, a two- or three-octave MIDI keyboard controller, by either M-Audio or Edirol, will do nicely. I have a M-Audio O2. All of these will control GarageBand's built-in synths. Some of these USB controllers are sometimes packaged with "lite" versions of pro software like Logic Express or Cubase; that'll give you a feel of what's possible beyond GarageBand.
These can be found at several places online, including Sweetwater.com, Musician's Friend, or Music123.com.