How to check how much RAM a program uses?
If you go to the Utility folder on your Mac, open the Activity Moniitor.
Then go to the Memory tab.
There you will see who is using how much.
There is also a Disk Tab, where you can also see who is using how much on active basis (when programs are running). You wont see the passive usage (how much information is stored) in that window.
In this example the application Firefox
Using Memory:

Using Disk space:

How much space is a Application occupying on the disk, is answered by looking in your Finder -Applications then under the - Size coulomb.
Again in my example, the Firefox occupies 147 MB of permanent disk space, which is not the same as the information above which was for the active situation.
Back to your original problem- slow computer:
In the Activity Monitor look now in the CPU tab to see who is most active there.