Can I move apps to internal USB storage?
Sounds like you need to clean up your internal memory first before you can do anything.  There's three ways I can suggest to free some up:


Clean out your app caches to free up space (browsers and maps are usually the worst offenders): you can use an app like SD Maid (it's tiny, you should be able to install it) or by going into the application management section (Settings-&gt;Applications-&gt;Manage Applications-&gt;All, then click on an app name) and manually clearing out app data for hogs like Google Maps and the like using the Clear cache button (note: don't clear the app data! just the cache).
Uninstall some apps, then reinstall them after you've moved other apps to the SD card.
Root your phone and uninstall bloatware / system apps you don't use with an app like Titanium Backup (make sure you back up the apps first in case you need them later).  Note: this will keep your phone from being able to install carrier Android updates in the future.


Android is usually pretty good about auto-installing apps to non-system storage when possible.  Does your version have an option to prefer installing to SD set?