It has to do w/ genus. I suggest Zebrasoma, Paracanthrus, or Naso genuses. The Acanthurus are not as hardy, and need mega-tons of space.
The most popular Zebrasomas are the Yellow, Purple, and Sailfin Tangs. All are perfect begging fish, and I suggest one of these.
The only popular Paracnthrus is the Hippo/Regal Tang, also a great fish -- tends to be fairly small.
The Nasos are really a genus, but the term Naso ushally refers to one fish, the Naso Tang, often called Orange-Spine Unicornfish. These tend to get big, and need lots of space, but have great personalities.
Acanthurus is prob'ly the largest group, but I don't recomend any of them for small tanks or begginers.
If you want more than one tang, it's advised that you pick your next one from a diff genus. So if you got a Purple Tang and wanted another, you could get a Hippo Tang. It's fairly easy to figure out, even if u can't remember the genus, you can tell by body shape, which is the same for fish of the same genus. Good Luck, Tangs rule!