There are 2 different kinds of cache--there's app caches, and there's the system cache. Clearing app caches is pretty easy--you can go to Settings/Apps, select the app in question, then tap "Clear Cache." There are also a number of apps out there that make this even more convenient--try App Cache Cleaner.
The system cache is where the OS stores temporary data, logfiles, etc., and is located in what's called the cache partition. You can wipe the cache partition without losing important data, and this can sometimes resolve issues like lagging other glitches, but it requires getting to your device's recovery menu. That's why it's important to know what device you're using.