Make sure your battery is charged.
(If the phone is already powered on, hold in the home and power button until it shuts off, then do the following)
If/when it's charged and powered off, hold in the round home button, and the power button on your iPhone.
Once it turns on and you see the Apple logo on the screen, let go of the power button, but continue holding the home button. Keep holding the home button until you see the iTunes logo on a black screen indicating for you to plug your phone into iTunes (This is recovery mode). (You may have to try this a few times until you get it to boot into recovery mode)
Once it's in recovery mode, plug your phone into your computer with iTunes installed on it, using the data cable. Then, restore your phone using iTunes.
Hope this helps :) If you need anything else/have any issues/questions/etc, feel free to let me know, and I will do my best to help you get this sorted out ;)