I assume you're talking about the rear wheel, since the front is pretty easy. One a bike like this, after you get the chain back on the sprocket and the axle back into the drop-out slots (make sure all the washers and such go back onto the correct side of the slots), you need to do three things at the same time:
1) tension the chain by pulling back on the wheel,
2) align the wheel such that it's centered between the frame forks, and
3) tighten the axle nuts.
Generally, you pull the wheel straight back with one hand and slightly tighten the axle nuts with the other. Then you can give it a spin the verify the alignment, and touch the chain the verify it's tight enough (leave just a little bit of slack), and if it's all good, you tighten the nuts fully (tighten firmly, but don't over tighten them).