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 phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
6. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
7. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
8. Then press the "Power" button to hard reset the phone.
9. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.