A cheap moisture wicking shirt from WalMart will work. I started out keeping everything in my handlebar bag. Now, I try to do without a handlebar bag--so the jersey is good for keys, cell phone and snack.

If you want a jersey, I recommend Cannondale jerseys from PerformanceBike.com. They don't fit so snug, but I would order at least one size up. Now is a good time to pick some things up on clearance.