Plugins and $_POST

I have a plugin with a widget. It's a simple contact form. I can't seem to pick up the results of the contact form's input with $_POST. Even though it sends the variables in the $_POST it comes to the script of the plugin empty.

How do you get around this in WordPress? Can someone give me some material to read or keywords to look up. I have tried for 2 days to find a resource on this but it's hard to search for anything with the words wordpress and post. All I get is post (blog) stuff.

Thanks in advance!