Sounds like you're either running an early version of Android of the phone originally came with an early version. Unfortunately, earlier versions of Android partition internal storage with a tiny partition for apps. Since it's tiny, it tends to fill up very quickly.
You need to:
- clear caches
- uninstall any apps you no longer need
- uninstall updates for apps you no longer need but can't remove
There are ways to re-partition and to move more apps to SD however they require rooting (and some techie skills). Note that rooting may invalidate your warranty. See the rooting section of your phone's sub-forum for more information.