Go to settings and check running applications. Check if any unwanted application is taking up RAM space. manually force close it. Your device should work fine.
You can also ROOT your device, which means getting an Admin access to your device core file system. This allows you to remove any apps that you think is taking up RAM spaces.
!! ROOT is risky, since it is getting you an admin access to core files, any alteration or modification can brick your device." If that happens you will loose the warranty. (If you are having any)!!