To see what files are hogging your megabytes, try an app like Id-design’s WhatSize.
Check vendor Web sites for apps you got elsewhere. Go to Apple > App Store and click Updates to check for, and then download, new versions of software from the App store (including OS X itself).
If force-quitting and restarting doesn’t stop your slowdown problems, you might need to do more:
Are you staring blankly at this chapter because you can’t even get your Mac to turn on? Try this: