What kind of armbar? There are a ton. Since you get to mount easily, you may want to learn armbars from mount. There are quite a few. You can armbar from side control, from guard.... Do you train somewhere?
Defending armbars is another story. It's hard to learn something like that from the Internet.
I definitely won't try to explain an armbar to you. My submission descriptions are terrible.
IMHO, Ironman is the best guy on here when it comes to grappling instruction.
Otherwise, go to lockflow.com, find an armbar you want to learn, and practice, practice, practice until you can do it in your sleep!