First, I would make sure you don't have a bunch of apps running at the same time. Just because you stop using an app doesn't mean it actually closes. It keeps using your resources until you actually close it.
Second, I would suggest cleaning out the cache of some of your apps. Especially things like your browser, Facebook, etc., which continually add stuff to the phone. Just make sure you back up anything you need first to access things in those apps like passwords, etc.
Third, uninstall apps you don't actually use and are just sitting on the device taking up space.
And fourth, I would suggest you check for viruses, malware, bad apps, etc. Locate and uninstall/remove them.