Even if you follow all the rules on keeping white clothes white, they can still become dingy or yellowed over time. So what to do?
The first step for washable clothes is to mix a solution of warm water and oxygen-based bleach like OxiClean or Clorox 2 or Tide Stain Release. Submerge your white garments and allow them to soak overnight. Then wash as usual, adding 1/2 cup distilled white vinegar to the rinse water.
You can also add bluing to the wash or rinse water to make clothes appear brighter. Just follow the directions on the bottle.
If neither oxygen-based bleach or bluing gets your clothing white enough, you can use a color remover to strip away any color from the clothing. It cannot be used if there is colored trim or decorations on your clothing.
If possible, dry the clothing outside in the sun. The ultraviolet rays of sunlight will help to whiten the clothes.
Specific questions? Just ask here.