Hi Ammar. Hold down the power button and both volume up and down keys until you get to the recovery screen;
Use the volume keys to select the top option (recovery mode), press the option button (the square on the bottom left of the navbar);
Using the volume keys again, highlight 'wipe data/factory reset. You'll get the usual warnings (it will return your phone to as new with all data erased);
Once you've selected it, the next screen will have an option 'delete all user data', highlight and select.
This is based on a stock A4, if you have a custom recovery (CWM or TWRP) then it may look different but will be self explanatory.