I bought Home Recording Software, but I need a MIDI controller, what is that?

Ok, i play guitar and my friend sings....we are trying to record our own songs, i know nothing about recording at home, but im willing to learn, but I would like to know how to plug my guitar into my computer and record the material....ummm....I hear people talk about MIDI controllers, audio interfaces......and I dont know what to get....Can somebody list specific products that will help me that. BTW, i bought Cakewalk Home Studio 7 Sonar...or w/e.....