Severe dehydration needs immediate medical treatment. In hospital, you will get fluids through an intravenous drip.
If you are mildly dehydrated, the best thing you can do is to drink more water. Drink small amounts of water regularly.
You can also drink oral rehydration solutions that you buy from your pharmacy. Or you can make your own rehydration fluids.
Avoid alcohol and caffeine, as they can make dehydration worse.
Recipe to make rehydration fluid
6 teaspoons of sugar.
½ a teaspoon of salt.
1 litre (5 cups) of boiled water.
Stir the mixture until the salt and sugar dissolve.