Fixing a disable iPhone due to forgetting passcode?

I tried multiple time and watched many YouTube videos but it won't work. IPhone 4. When it's downloading it just turns on and something pops up on my computer saying that I must enter the passcode first (which i don't know) I'm trying to fix it for my friend as a birthday present. It only downloads a little bit.

You must connect it to ITunes and reset it. All information it contains will be erased.