Turn it off by holding down the power button for several seconds. Turn it back on but hold down the VolumeUp button before pressing the Power button.
Keep holding them down until a menu shows up then use the Volume down button to scroll to the Factory Reset option and use Power to select that option. You may have to confirm - do so as instructed.
You'll lose all your files and personal data but you already have by the look if the situation. Find some strong security software from the Store when you're up and running. I use Lookout which is free and quite effective.