Singlespeed Chain Tensioning

The chain on a singlespeed bike needs to be tensioned manually since there's no spring loaded derailleur to automatically add tension. If you find your chain too loose after installing your rear wheel, here's a quick way to tension it. First loosen both axle nuts. Now angle the wheel in the dropouts so that it's closer to the non-drive side:

 

Now tighten the non-drive side axle nut only. Next you should pull the wheel toward the center of the frame and tighten the other axle nut:

This will increase the tension on the chain. Check that the chain doesn't bind as you turn the pedals. If you feel any extra resistance then loosen the non-drive side locknut and retighten it again to relieve some of the tension. A binding chain will likely snap if you ride it like that. Generally you want a little bit of slack, but not so much that the chain can actually fall off by itself. Here's a decent setup:

 

Also keep in mind that chainrings usually aren't perfectly round so a chain that appears loose in one spot can still bind as you turn the pedals. If you're lucky you can get the perfect chain tension in your first try, if not then you just have to play around with the locknuts a bit until it's right.