Like the guy above me said, your wheel most likely just needs to be trued. If you have patience and you're good with your hands you can watch videos on Youtube on how to do that. You need a simple tool to do that, though.

Also, make sure your chain doesn't have rusty links. They too can cause your chain to skip. Inspect them one by one to make sure they all move freely and not stuck. I once had a bike that used to skip and I thought it was a shifting problem but turned out there were a couple of links that were stuck due to rust. Cleaning and lubing them took care of the problem.