Android is actually quite sophisticated about memory management. It will keep recently used apps in memory so that when you access them again, they're ready, and you don't have to wait for them to load. If the memory in use by an application that isn't active is needed by a newly accessed program, Android will free up that memory for use by the new program. With this design, most of the memory will show as used. That doesn't mean it's not available for another app when needed, though. It means Android is working as designed.
You can "kill" apps to free up memory in the Application Manager function under settings, but it won't really benefit you. And it may slow down your user experience, if the phone has to repeatedly reload the applications you use.