Caring for indoor plants is easy, just replicate the conditions in which they naturally grow! Okay maybe it isn't that easy but common sense in the position and watering will reward you with a long & healthy plant life.

Water: Where possible remove single plants (in there plastic pot!) from the waterproof container. Either immerse the pot in water, for plants such as hydrangeas, or sit the pot in 1 or 2 inches of water for African violets etc. Add liquid plant food, such as Baby Bio when required. When the plant has had enough water it is very important to allow time for the excess water to drain from the pot. If the plant is left sitting in water it WILL rot the roots and kill it.
For planted arrangements or where the plant can not be removed from its container the best way is to regularly check the soil. Move enough covering, moss, stones, bark etc. to push a finger about 1 inch into the soil, this should lightly stick to your finger if it's too dry increase your watering, or too wet allow the plant to dry and decrease frequency of watering. Most plants appreciate a mist spray.

Position: The general rule is a bright spot. Some foliage plants sansevieria (mother-in-law's tongue), aspidistra and many varieties of dracaena are happy in darker positions but flowering plants in particular will not do well. Window-sills do pose a problem though. In the winter with the curtains drawn the temperature can be too cold for delicate plants and in the summer the midday sun, magnified through the glass, can scorch the plant.