Most stains can be removed by simply washing the item in cold water. If necessary, you might want to place a drop of liquid detergent on top of the stain and let it stay for about half an hour before washing the item as usual. Heavier stains require more work, but most can be eliminated, even if it takes several goes at it.
For heavier or large stains, presoak the item in a bucket of water mixed with liquid detergent. Stir every few minutes and let it sit for about 30 minutes. If the stain is fresh, you might want to try and absorb as much of it as possible before you put the garment into water. To do it, simply place a cleaning cloth on top and press on the stain. Bloat repeatedly over the stain in case something remains.
Some stains may require commercial products. For example, there is a special ink remover on sale that will work better than simply using soap and water.