I live 10.4 miles from work. I'm a new rider as well. New like in the last 2 months give or take a week. My route is very up and down hill and some of the ups are quite extreme. Because of that I try to go as light as possible. But I do take along things that I found out were a complete and total neccessity and I found this out the hard way. Tire irons, a new tube, a quick patch kit, and the means to inflate that sucker. I found myself halfway to work with a rear flat. Out where I live, it's normal to see people pushing their bikes. Remember the hills? So it took some time before anyone even asked if I was having trouble. And when someone did, it was because I decided to try a little experiment that seemed to work. If you break down and can't get yourself going again. Don't push bike. Pick it and carry it. As soon as I did that, I had a ride in minutes.
Other than tools and tubes, I really only take a full water bottle and some picture ID in case some moron crams into me.
One question though. I don't have those cool guy shoes that lock onto the peddle. What are the advantages of those over the toe clips?