A chain is made of steel, and eventually that steel just gets streched out from an engine pulling it arount and around. Its just happens.
To adjust it you will need a rear stand (either buy one or find someone with one). Put it up on the rear stand, loosen the axle nut and give it a little tap it so it isnt scratching against the swingarm when you tighten the chain, loosen the outer nuts at the ends of the swingarm and tighten the inner ones. It will pull the wheel back and tighten the chain up for ya. Then check the tension in multiple places by rolling the wheel (while still on the stand, of course). It should only move 1 inch total (half up, half down) in the middle. When thats good, crouch behind the bike and rooll the wheel again. Look at the top of the chain and see of if it is angles of turning a bit. It shouldnt. All those bright shiny pieces in the middle should be in a perfectly straight line while you are turning the tire. That aligns the tire. When thats all done, tighten the axle to 75 ft.lbs., and tighten the nuts at the end of the swingarm.
Excellent tip on adjusting the chain tension. One thing to mention is that if the chain is too worn out, no amount of adjustment is going to make it tight enough, and you'll need to replace it. If you do end up replacing it, you might want to consider replacing the sprockets as well.
In addition to stretching over time, a chain will "loosen up" if you do a lot of wheelies or anything else that puts more strain on the chain (hey, that rhymes).