Check the garment care label. If the manufacturer provided any special care directions (and most manufacturers always do), they're going to be printed on the label. If the care tag says to only dry clean the shirt, you must take it to the dry cleaner.
Spray a heavy coat of non-acidic stain remover on a stain, and let the remover soak in for about 10 minutes. Fold a paper towel and place under the stain. Use a second paper towel to dab inward from the outer edge of the stain. Spray the spot and let the stain remover soak in again if the first soaking does not remove the stain.
Fill the washing machine with cold water, unless the shirt's care tag suggests using a different temperature. Cold water is the least likely to make your shirt shrink. Add one cap of laundry detergent, then put the shirt in the washing machine.
Listen for the wash cycle to end, and the rinse water to fill. Add one cap of liquid fabric softener to the filling rinse water. Let the washer finish cycling.
Air dry the shirt or dry it on the lowest possible heat setting of a dryer. Excessive heat can shrink cotton shirts.
Check the dry shirt for wrinkles. Spray a light coat of starch on the shirt, just one section at a time, and start ironing over the starched section. Keep going until you have finished ironing the whole shirt. Keep the shirt on a hanger until you are ready to wear it.