First off, when the manufacturer announces that you have 16 GB of available memory, you actually only have 12 or 13 at your disposal. Why? Simply because your Android also needs space to operate with too.
To check what's taking up room on your smartphone, take a look at the storage tab in your settings. The important information to take a look at is here: Device Memory.
How to recover memory
Everything takes place in the settings. More specifically, the action happens under the applications tab. Depending on your device, its either called application manager or simply, applications.
Once you've made it this far, it's a matter of common sense. The cache is a kind of library, in which the applications will store data that they'll retrieve later.
The application page
To help you, here is a guide on how to use an application page.
There's not too much to explain, it basically removes the app. Don’t do this if you still plan on using the app, obviously.
Move to SD card
This option will allow you to move entire applications to the SD card stored inside of your device. This might get a little complicated, but that's something we will tackle in the near future.
This will clear up all of the data related to an application. If you erase the files for an app like Facebook, you'll have to re-enter the passwords again. This is something to use with precaution. For example, saved games will probably also get erased.
Empty the cache
This is also a domain that requires common sense. In fact, what we are about to do is delete the cache for certain apps that are taking up a lot of room. The cache, like mentioned before, is a type of library where your apps store their files to retrieve later on. It can be emptied from time to time.
Emptying the cache will allow you to deblock an application that doesn't work anymore, like Google Play after its failed update.
In conclusion, you can uninstall apps, empty the cache and completely erase the data for an entire application. This will help your Android device run smoother and free up some space for new downloads.