You don't mention what phone you have. Depending on the manufacturer, the "skinned" version of Android could be taking up a lot of space. That, coupled with "bloatware" (the crap that the carrier installs that you can't uninstall) tends to take up a TON of room.
Fear not - there are solutions. Quickest and easiest is to clear the cache. Step by step is different for every phone, but if you go to settings, see if you can find something about "storage". you should find a breakdown of what's using up all your storage space. Once you see "cache", tap on it. It will ask you if you want to clear it, press ok/yes.
Also, download "CleanMaster" from the play store. It's an easy to use app that allows you to clear some unnessisary files from your phone including any leftover bits of apps or games you've previously deleted. You can also disable any apps that you're not using - it doesn't gain any space but at least they're not running and using up CPU cycles.