It's difficult to come up with rules that fit every orchid plant because there are so many different kinds with so many diverse needs. The most important thing you can do is to find out what kind you own and then to try to follow the rules that apply to that particular orchid. However, there are some general rules that apply to most orchids.
First of all, orchids, in general, do not like to have wet feet. In nature, water runs heavily over an orchids roots and then, when the rain stops, the orchid roots and leaves dry quickly as the roots are not planted in the soil but exposed to the circulating air. In your home or greenhouse, you need to make sure that you drench your orchid about once a week and then let it dry for the rest of the week. In the winter you might want to extend your watering cycle to every ten days. You should also mist your orchids but make sure that they don't stay wet. How often you mist will depend on the humidity in the orchids environment. If you mist too much or too often water will stand on the leaves and in the crotches of the leaves and the orchids crown and this is conducive to the growth of fungus. If youre not sure about water, water and mist less rather than more.
Generally speaking, orchids like light but they prefer indirect light. If you keep an orchid in strong, direct light its leaves can become sunburned. If your orchids leaves look very dark green, the orchid is most probably getting too much sun. Just move it back from the source of light a bit and see how it does. A sheer white curtain can act as a good filter is you want to keep your orchid closer to a window.
All orchids need fertilizer. The rule of thumb is to fertilize weakly weekly. In other words, use one quarter to one half strength fertilizer and use it weekly. Fertilizer that is formulated for orchids is a good choice. A 20-20-20 plant food also works. Just make sure that whatever fertilizer you use is high in nitrogen.
Orchids also need to be repotted about every two years. Be careful not to disturb the roots too much when you remove the orchid from the pot. Remove any bad roots (brown and mushy) with a sterilized tool and remove as much of the old potting medium as possible. Repot the orchid in suitable potting medium into a pot that is slightly larger than the orchids old pot.
And thats about it. Of course there is a lot to know about orchids and these are just some general rules. Don't forget to learn about the type of orchid you own.