Here are some tips to help you choose the correct size. If you don't have a measuring tape, you can use a piece of string and then measure it with a ruler.

Chest
Raise your arms slightly and measure around your body (it's best to do this without a shirt). Make sure to go over the fullest part of your chest.

Neck
For casual shirts, find a well-fitting shirt and lay it flat on a table. Measure the "top" edge of the collar. For dress shirts, measure around the base of your neck, leaving one finger's width between the measuring tape and your skin.

Sleeves
Place your hand on your hip with your arm bent at a 90-degree angle. Measure from the middle of the back of your neck, over your shoulder, and down to your wrist.

Waist
Measure around the narrowest part of your abdomen.

Hips
Measure around the widest part of your hips.