Using a WP Path in a Plugin

This is probably a basic question for most of you plugin programmers out there, but I’m a wordpress plugin noobie, and I can’t seem to Google my way out of this problem.

How do I use the wordpress path variable in a plugin? I’m looking for something like “stylesheet_directory” or “template_directory” but for the plugin. Whatever I try, it seems to end up being blank. Do I have to include a specific wordpress file at the top of the plugin or something?

Thanks in advance.