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

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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...
 
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