How to replace stripped spoke nipples?
You need a spoke key of the correct size. See Which size spoke tool. Currently size 14 for some discussion - the short version is either visit your LBS and buy the size that fits your spokes, or just buy the three most common sizes.



Using a screwdriver is unlikely to work - wheelbuilders use them in the very early stages when the spokes are very loose, but as soon as a little tension goes onto the spoke the nipples are impossible to turn that way.

Edit: a couple of links on how to true wheels. If you're willing to spend money The Bicycle Wheel is pretty definitive and it's quite a short book. It's hugely better than anything I've seen on the internet.

And your LBS should have a pile of spare nipples for their wheelbuilder(s) to use. They're only a few cents each, but buying retail like this probably 10c-20c is more likely.