Unless it is the middle of summer and there has not been enough rain, watering your outdoor plants is usually not necessary (or not very often). But indoor plants rely on you as a source of water and extra nutrients in the form of fertilizer. It is important to know the individual water and nutrient needs of each plant to keep them healthy.
As mentioned, individual plants will require different amounts of water to keep them growing optimally. But what all plants do like is moist soil. If you are worried about over-watering your plant, make sure that the pot you choose has a good drainage system. With holes in the bottom of the pot or gravel inside the pot the soil and plant will soak up the necessary water and the excess will run out through the bottom.
If your houseplants are not thriving no matter what you do, there are two things to look into. If you are using tap water to water your plants there may be too much chlorine or salt present. A solution to this is to use distilled or filtered water or you can leave a container outside to collect rainwater. Either option is acceptable and may be the change you need to make to grow healthier plants.
Choosing to fertilize your plants is another way to give them a boost. Fertilizer contains nutrients and elements that plants need to grow. Indoors plants do not need as much fertilizer as their outdoor counterparts do. Because of a slower rate of growth, feed your plants minimal fertilizer. In the winter time you can probably skip this step altogether. The spring or summer time is the best time to fertilize indoor plants. This is during their growing phase when they need the extra nutrients the most.