All plants need water and learning how to water your weed effectively is a key part of being a grower.
Plants draw water from their roots and give out water vapour into the atmosphere through their
leaves. If plants get too little water, they're unable to utilise all the nutrients they need and if they are over watered they basically drown through being unable to absorb the oxygen they need through their root system.
Judging the right amount of water to give
Possibly the best “rule of thumb” for watering is the “rule of the first knuckle”, in other words, stick a finger into the soil down to the first knuckle. If it feels dry add water until you see water running out of the drainage holes in your pots. To avoid mould you'll need to clean up this water. For this reason you may find it helpful to stand plants in trays which you can then empty. While the soil is moist the plant still has water it can use, when it dries out the plant needs more. If the soil dries out to the point that it separates from the container then the plant is in severe need of more water.
The key to successful watering is good drainage
Even experienced growers can benefit from the use of “smart pots” which are basically designed to get oxygen into the plants through the roots. They are a great help to beginners as they make it a whole lot harder to over water a plant. If you use ordinary pots then make sure they have sufficient drainage holes. Getting the growing medium right is also a big help. The best medium is composted super soil with a little perlite mixed in. Avoid bark or wood chips they are terrible for growing cannabis.
A note on seedlings
It's a good idea to keep small plants in small pots, since it makes it less likely that you will water for the size of the pot rather than the size of the plant. If you must keep seedlings in bigger pots then aim the water at and around the plant itself. It's better to water little and often rather than in floods. This again, helps to avoid over watering.