how do i know how much memory is on my computer???
To check the amount of memory (RAM) on your computer depends on your OS. If you're running windows, you could download a utility such as CPUz ( http://www.cpuid.com/softwares/cpu-z.html ).
If you're talking about how much storage (Hard Drive space), then simply opening My Computer should display how much is used/available on each drive.
If this doesn't answer your question, add more details, mainly which OS you run.