First try to delete files from smartphone manually. Go to the file menu of your phone and delete all the files that are not system. Also, delete all programs using Uninstallers in memory of the smartphone. In case if your smartphone has a flash card, format it.
You can also reset the firmware of your cell phone using a special code. You can find codes to reset the firmware in the Internet but the most reliable option would be to use the code obtained from the manufacturer of your mobile. Find his site in the technical documentation for your phone. Call the contacts located on it, providing the IMEI number of your phone. Enter the code.
Synchronize your phone with your computer. As a rule, data cable and a CD with the necessary software is included with your phone. Otherwise, you'll need to find these components yourself. Buy data cable in shop of cellular communication. Not necessarily with him was a CD with you can easily download it from official website manufacturer.
Install the software and driver, connect the phone to the computer and make sure that "sees" the phone. Then navigate to the phone via PC and remove all the files which are not system. Delete all the files located on the memory card, or format it. Do not disconnect the phone from the computer until the operation is complete, and after it ends, restart using synchronization software.
To prevent cluttering the phone, your best option might be save and uncompress files on the memory card. This will help not to clutter up the smartphone and, if necessary, to quickly get rid of excess files.