Most single speed bicycles will have horizontal dropouts. You apply tension to the chain by loosening the axle lock nuts and pulling back on the wheel. Obviously you tighten once you achieve the tension you desire.
It's kinda tricky, and there are a variety of ways to do it. Its always helpful having an extra set of hands to hold the wheel back for you. If not, I prefer a se saw method where you pull the wheel back where you want it and angle one side, to tighten the chain more, and tighten the lock nut. Then you have a loose side which you press over to get as close to straight as possible, and tighten the lock nut. Odds are theres too much tension on the chain now and the wheel isnt aligned straight. So you loosen the lock nut that you had angled ever so slightly until the wheel comes into alignment.
Hope that makes sense.
Heres what a horizontal dropout looks like.