If you are only going a couple of miles, and only occasionally, you may be able to get by with a backpack, but for all the other reasons listed here, you'll want to get a rack and either panniers or a rack bag. I'm much more comfortable hanging my stuff off my bike instead of me.

Start with what you have, and if you like commuting, get a rack and bag/panniers. The only wrong action is to not try commuting :-)