Based on habitat there are basically two types of orchids; epiphytes and terrestrials. It is essential to know whether the orchid you are attempting to grow is an epiphyte or a terrestrial, as requirements of both are different.
When growing an epiphyte, try picturing in your mind the ecological conditions prevailing on the tree branches. Availability of water and light, temperature fluctuations and air movement are the factors controlling the growth of an epiphyte on the branches.
Water: The only water available to the orchid on the branch is the rain water, which these plants have learnt to harvest. There are many orchids which possess specialised roots, having velamen- a kind of spongy tissue, which swells up upon absorbing water. In dry state it is dark green in colour and turns light green on absorbing water. Some of the other orchids form nest-like mesh with their thin roots. The orchid uses the water collected in these meshes as well as in the bark fissures.
From the above information you can make out the amount of water which is naturally available to the epiphytes. So water is a major issue which is misunderstood while growing epiphytic orchids. Generally people think that being tropical plants, they would be requiring huge amount of water and consequently end up literally drowning the poor plant. When actually, in between the rains, these plant are living in almost xerophytic conditions!
There are two things to be kept in mind while watering; keep the roots moist but not wet and take care of drainage. Frequency of watering should be, in general, about three times a week in summers and only once in a week in winters. It is also advisable to inquire watering details before purchasing, as requirements vary according to species of the orchid as well as on-going growth stage of the orchid.
Water logging should be avoided at all costs. Remember epiphytic orchids never face water-logging on the tree branches, it is rather the other way round, they mostly face drought like conditions until the next rain!
Epiphytes may not survive high watering, but love and thrive in high humidity. If the humidity is not on the higher level in your area then put up a humidifier. There are also some cheap ways to increase humidity around your orchid. First, put your orchid pot in a flat plate-like retainer. Now fill this retainer with water. If you have large number orchid pots, then you can keep a bucket full of water in the vicinity of your plants. By these two ways you can increase the humidity within the micro-climatic zone around the plants.
Watering can be done either by pouring with hand or by spraying. Remember the rainwater drips on the orchids through the canopy. Some orchidists also advise dipping the roots part (only) in the water. Keeping the orchid outside, like letting it sit on the grass for the night, so that dews collects on it is another method to provide water (but before putting your orchids there, make sure that nocturnal animals won’t harm it).
•Air Movements: Epiphytic orchids are naturally inclined to grow in high humid conditions. This is aided by good movement of air at the canopy level. As it normally rains heavily in the tropics, high humid conditions can be quite oppressive but this experienced more at the ground level. The air around the branches does not remain stagnant for long, which not only increases the rate of transpiration (rate at which the plant loses water from specialised pores called stomata), but also results in temperature fluctuations. While growing epiphytes, please try to maintain good air movement. This can be done by hanging the plant in an open place, or when growing inside your house or in a greenhouse you can use a ceiling fan (at slow speeds) or a desert cooler can also do the trick. Desert coolers also help in maintaining higher humidity.
•Temperature levels: Most epiphytic orchids flourish in warm to hot temperatures (from 50˚F to nearly about 113˚F). Hot conditions accompanied by high humidity are a sure recipe from good plant growth, provided that diseases are controlled. If you are located in colder climates, then it is best to maintain a greenhouse or keep your orchids inside where temperatures are controlled. Please remember, while keeping orchids under controlled temperature it is important to keep a check on humidity levels, as these tend to decrease in such situations.
•Light requirements: As most epiphytes are found in tropics, they love the sun. Most of these grow in partial sun light which is available through the canopy of leaves. So do not put your orchids in full sunlight. Though there are some orchid species which can tolerate full sun, most will dry out and eventually die. Best way to provide your orchid sunlight is to keep it at a place where the morning sun shines the most. Avoid afternoon sun at all costs, as this can burn your plant.
•Feeding your orchids: In nature epiphytes grow in conditions where nutrition is scarce. They make do with whatever they can extract from either the dust in the air (mostly probably) or through the percolating water which brings along nutrients exuded by the tree leaves or the bark. This, as you can imagine, is in very small quantities, so when providing fertilizers to your orchids don’t overfeed. It is best to fertilize once in a month with a fertiliser which contains not only Nitrogen (N), Phosphorus (P), Potassium (K), but also micronutrients like Magnesium (Mg). You can also mix the fertiliser in the water when watering or in the spray pump.
•Growth medium: Growth medium is the substrate on which you grow your orchid. You can use anything from sphagnum moss, activated charcoal, to even the simple coconut husk, as the growth medium; provided the medium can store water and is porous enough to keep the roots dry in-between the watering periods. It is also advisable to keep a check on growth of fungus or any other pests in the medium.
While the above information has been generalised for epiphytic orchids, you should also inquire specie specific needs from the supplier.