Do you mean that your hard drive is almost full? There's a difference between storage space and memory (RAM)...
Take a look at ds store's article - Why is my computer slow? - some tips there for almost-filled hard drive space.
You probably need to delete some items from your boot drive. If you want to keep some of those items, invest in an external drive to store user information and the like. You may also want to invest in a new, higher-capacity boot drive.