Memory Usage 100%
 

My CPU usage is always 100%, I don't know why. It also says that I have 503 RAM when I have 512. What's the matter?

no doubt your ram is fine ...
read this to see what i mean ...

as of the cpu usage, type the command :
top
in a terminal and look for the culprit, by checking out the column for cpu usage, and then look in the last column to see whats causing it ...

oh and if that thread i linked regarding the ram is not the case, then you should see in 'top' what is sucking up what percentage of ram ...