It's important to make sure your plant gets enough water to grow healthily, but not so much that it drowns.
There is no universal answer for how to water plants, but there are some general guidelines to follow. Specific watering instructions can be found on your plant's care page, but as a general rule, consider where your plant lives in the wild. Desert plants are accustomed to going without water for extended periods, whereas tropical rainforest plants are accustomed to frequent showers and high humidity.
You should water almost all plants only when the top inch or so of soil feels dry. The finger dip test is an easy way to see if your plant needs watering.
Never give in to the temptation to over-water. Overwatering is just as bad as underwatering. Allowing a plant to rot in water is dangerous. One of the reasons we recommend keeping your plant in its nursery pot (the brown or black pot it comes in) is that nursery pots have drainage holes in the bottom. The sink is the best place to water your plants. Water them until water runs out of the holes in the nursery pot's bottom, then leave them in the sink for about 30 minutes to let any excess water drain. Then place everything back in its decorative pot.
A plant that is underwatered will typically have dry, brown or yellow leaves. Overwatered plants frequently have yellow, drooping leaves.
How to keep your plant watered when you're away
If you're going on vacation and worried about leaving your plant behind, there are a few things you can do to keep it alive.
Water bulbs slowly drip water into the soil of your plants to keep them hydrated. Others allow the plant to absorb water as needed - look into hydrospikes for this. Try putting one end of a damp piece of cloth in the soil and the other end in a glass of water for a do-it-yourself method for small plants. As the plant requires more moisture, it will absorb it through the cloth.
However, the best solution is to have a friend or neighbour come in and water your plants for you. Simply instruct them to perform the finger dip test.