how do i check my ram usage?

go to Applications -> Utilities -> Activity Monitor. Click the tab which says System Memory. It should look something like this;

 

There's a widget as well that provides a more basic view of the info in a neat package. I can't remember the name of it, but I found it in the OSX section of Apple.com, so you can find it there too.

Also, I use an app called MenuMeters. It's a lightweight app that displays basics of your CPU usage, memory, and network in the menubar (sandwiched nicely between Airport and Volume). Google it, it's really all I use and it's very convenient...

 

I use iStat for all my system monitoring needs. It's great

 

What do each of the memory usages mean? Such as Free, Wired, Inactive, Active, Used. How do I decipher that?