There are three things you can do to tighten a cotton shirt: shrink it, sew it or tie it. If your shirt is made with pre-shrunk cotton, you won't be able to shrink it too much. Sewing it is probably your best option, assuming you know how to. If you're a female you could tie the extra fabric together and make your shirt take on a whole new personality, but you may find this is too revealing. If all else fails, have a seamstress alter it.
Set your washing machine for a small load using the highest water temperature possible. If the water in your home never gets too hot, you can add boiling water to the washing machine.
Let the washing machine fill up.
Add laundry detergent as usual.
Place your shirt in the washing machine and leave it there until the wash cycle is complete. Do not add any other clothing.
Place your shirt in the dryer on the highest heat setting you have.
Turn the dryer off once the shirt is bone dry.
Let the shirt sit in the dryer until the fabric cools.
Determine how many inches tighter you would like your shirt to be.
Turn your shirt inside out.
Smooth the shirt out on a flat surface so that there are no creases.
Draw two lines, each one parallel to one of the shirt's side seams. The spacing between the lines and the seams should equal the number of inches you want taken off the shirt. So if you want the shirt 2 inches smaller, the spacing between each line and seam should be 1 inch.
Place six to 10 pins along each line, going all the way through the shirt.
Cut the side seams off your shirt.
Sew your new seams, removing the pins as you go.
Put your shirt on.
Stretch the fabric around your torso and gather it together near your tailbone.
Wrap an elastic band around the excess material to form a tail.
Stuff the tail under the shirt, rolling it up until the elastic band is hidden.