Default [Q] Phone gefting slow and virus affected

Before i connect my phone with usb via otg everything was fine. After connecting with otg phone became slow and in gallery if i click on an image some other image is getting loaded. Launcher stops unfortunarely. Battery hardly gives 18 hours. I am planing to do reset. Wat all things nees to be taken care to have a perfect reset without any problem. Other than reset wat is the solution? Plz help me out