Especially if you are buying meteorites online, I strongly recommend that you read a good book on meteorites and then purchase a few inexpensive specimens that are at least 1 gram (if stony) or 10 grams (if iron) in size. You also may find this glossary of meteoritic terms useful. If the vendor is a member of the International Meteorite Collectors Association (IMCA), they will list their IMCA member number and the items that they sell are guaranteed to be genuine, i.e. you have some recourse if there is an issue.
The vendors listed here have web sites or eBay stores and I have purchased meteorites from all of them.