Sounds like you have either a virus, malware, or a bad app on the phone. You will need to figure out which and remove the cause to fix the problem.
If, in the end, you can't find or remove the problem, then you will probably have to "Hard Reset" the phone. This should only be used as a last resort though as it removes all data from the phone and starts over. Before doing this you would be best to back up all your personal data to a google account or a computer (files, contacts, music, etc.). Just remember you can't back up apps. So any data you want to save from an app you will have to locate and save the info itself off the phone. The app would have to be reinstalled after the reset.
NOTE: Doing this will clear 'all' the data on your...