As you become involved with growing orchids there will come a time when you will consider starting growing some species orchids. Fortunately they are now widely available from reliable dealers who produce plants from seed in the laboratory rather than the old fashioned way -- gathering them from forests and destroying native populations. From a conservation standpoint, always inquire from the vendor if their plants are seed grown. Reputable dealers will always tell you the truth. Recently there were native American species on sale on eBay which had been illegally gathered. When notified, eBay removed the listing.
A good way to start is to look at the plants you are currently successfully growing. What grows best under the conditions? See if you can find the parent species of these plants. Chances are that they might do well for you too. Consider where you live, what are the natural conditions there? In the US it is a mistake to try to grow cool growing genera in our southern parts unless you are willing to spend a great deal of money and work keeping them cool. It is much better to concentrate on the warm growing varieties that will love your natural climate. Conversely, growing Vandas in the northern areas is difficult.
How much light are you able to provide? Some species require high light to bloom and others are happier with shadier conditions. Can you keep high humidity around the plants? Do you grow in a greenhouse, windowsill or a terrarium? The lovely Jewel Orchids are grown primarily for their velvety, multicolored leaves -- but they must have high humidity.
Most enthusiasts concentrate on one genus as a specialization just because space is an issue. There are more than 30,000 kinds of species of orchids. It would be impossible to try to grow them all. So specializing in say, Cattleya species narrows down the number to approximately 50 species with varying growth requirements. Dendrobiums, on the other hand have approximately 1400 species so further specialization would be required unless you have lots of space. A great place to gather information on species growing requirements is The Orchid Species Culture pages. There is a subscription service which will allow you to purchase native habitat information on many orchid species. It is an invaluable resource for the series species grower.
A word of caution on something that has happened to me … be careful buying plants from the opposite hemisphere. If plants are imported from say the south and you live in the north, you are getting them out of their natural summer/winter rhythm of growth. It will take several years to acclimate them to the new location. I personally have had better luck purchasing the same plants from a vendor who has grown them from seed in the same hemisphere so they do not have to make the change. This is not to say it is impossible, but just be aware of the possible consequences.
Check out my Recommended Orchid Vendors for reliable seed grown plants. And as always, please mention that you heard of them from BellaOnline Orchids!