Orchids are easy to grow. They can grow in almost anything. Important things to remember is that they can grow in all conditions, but it is hard to force a cold tolerant plant to grow in high heat and vis versa.
Chose the plant that will like the growing conditions where you intend to keep it. If you are not sure ask us. Drop as an e-mail and we will give you individual attention.
Orchids have been traditionally grown in Fir Bark. It does not decay fast and is disease and pest resistant. It fills the need for a Cattleya to dry out between watering. Too much wetness encourages rot.
Sphagnum is another popular medium especially with commercial growers. It retains water longer producing more even moisture around the roots of plants that prefer not to dry out. Phalaenopsis and Paphiopdeilum are examples.
Both are excellent but not for everyone. If you like to water a lot, do not chose sphagnum. If you want to be able to ignore watering avoid medium that dries fast. You can custom mix different medium to get different water retention characteristics.
Coconut husks and coir are popular in Hawaii were they are cheap and available. Quality can vary greatly and it is not the best choice for most hobbyist.
All natural medium eventually decays and must be replaced. Usually every other year for most Orchids. There are new growing mediums like hydroponic stone, that is very successful with Orchids. It is inorganic so it does not decay. It can not retain more than 8% water so overwatering is very uncommon. It never packs so air circulation is always good. It wicks water evenly throughout the pot.
In time you will determine which works best for you.