loosen both nuts on the rear wheel. slide the wheel back until the chain is not too loose and then tighten one nut up just a little bit. then the other. make sure wheel didn't move. , also look to see that your wheel is centered in your frame down by the bottom bracket, if it's not loosen nuts and adjust and do all over, also make sure chain isn't too tight either.
once you get it, tighten both nuts in increments until the nuts are tight.
make sure your chain isn't too tight.
then if you feel real adventurous, get a rag that you don't mind being stained and rub all the grease and dirt off your chain. then put a drop of lube on each roller in the chain. rotate cranks a few times then wipe excess grease off.
make sure you don't get your fingers caught in your chain when rotating the cranks.