If it's a new stain you may be able to ge t it out with warm water and ordinary detergent. Vinegar mixed with water can also work. If it's dried or old, special stain remover bars can be effective. Wet the area with cold water and rub with the bar for a minute or two, then rinse. If the stain is still there you can soak the fabric in cold water overnight, having first rubbed it with the bar or put a stain remover solution in the water. There are lots of these available; I've tried Oxy stain remover (there are several brands) and they can work well, both by soaking the fabric overnight or by putting a spoonful in with your machine wash - sometimes you may have to do both.

Or you could try home remedies for older coffee stains, such as baking soda sprinkled onto a damp cloth and rubbed over the affected area, or a stronger solution of white vinegar than for fresh stains.