Press both the volume down & the power button at the same time and hold till the 1st screen appears,then let them both go.if it doesnt work try with the volume up button & the power button together as most phones will take you to the factory reset menu this way.
It should take you to a menu that you can select to factory reset.The volume buttons scroll through the menu items & the power button executes the menu item selected.Please note that you should have your phone charged enough to not go dead during the reset,the process is less than 1 minutes time.