When a plant is young, with a shallow root system, it doesn't need much water-only enough to supply the roots with water. Some people make the mistake of pouring water in until it runs out the bottom.
While that isn't bad for a big plant (in fact it's good, it prevents salt build-up), it isn't good for a small plant.
The roots of marijuana need to dry out a bit between waterings. A good way to judge if it needs water is stick your finger all the way into the soil a few inches away from the plant (carefully to minimize root damage). Water only when the tip of your finger remains dry.
Eventually you'll get in tune with your plants. You'll be able to tell by picking the container up a bit, or by the soils appearence at the surface, or how much the plant usually needs.
More on water:
For a beginner, overwatering and underwatering both make the plant wilt. Only with experience will you be able to tell which is which. The main difference is underwatering causes a brittle limp, overwatering causes a the same kind of wilt but not dry/brittle.
Even More:
The bigger the plant, the more water it needs. When it's young the plant uses a medium amt. of water. In full veg it uses lots.
It gets even thirstier during the first 3 weeks of flowering, when the plant more than doubles in height.
As flowering progresses the plant stop growing as its energy goes to resin/bud production. Water needs drop off drastically. And during the last few weeks of flowering the plant needs very little water.
It's a learning process. But soon you and your plants will be in synch.
