how to view memory usage?

i'm looking to be able to look at how much memory each application or program is taking up. To have a better idea of what to delete, move to a external, etc. thank you and have a nice day.

Memory (i.e. the RAM required to run the application) ? - for this start up Activity Monitor
or
Disk Space (i.e. the space required to install and keep the application) ? In Finder open up the view options, tick calculate all sizes then hit Use As Defaults.
Now you'll be able to see file and folder sizes in Finder.
Though I would turn this off again once you've done what you need to do

under view i see : view icons, list, colums, cover flow, clean up, clean up by, arrange by, show path bar, show status bar, hide side bar, hide tool bar, customize tool bar and show view options.

Right click an empty space > Show View Options > check "Calculate all sizes."

No (to post #3), he means Show View Options. Hit command-J on your keyboard to open that while in a finder window. The reason two topics were addressed is because many people use the word "memory" when they mean disk space. If you mean memory, go with the first answer, if disk space, the second.