You didn't specify what phone or what version of Android, so I cannot give you specifics. In general, research "recovery boot" for your phone model. Follow the instructions (you may even find an instructional video) and once in recovery mode, select Clear data (may also be called "Wipe data," or may even say "Factory reset") and then select Reboot from the recovery menu. There may also be an option in your phone settings, under Accounts > Backup and reset.