Just a quick addition to the other responses (which are right on the money, btw), but for now, a quick rule of thumb is that as you change chords, your thumb will follow the base note, and your other three fingers will keep picking the same three strings. Eventually you'll want to mix and match to your heart's content, but for now, this will give you a consistent sound. Lisa's lesson's are great... just follow each one, step by step, and don't move on until you've got the current lesson down. She gets into some really cool stuff towards the end. Good luck!