What do you do if you forgot the passcode on a new iPhone?

My mum bought me a new iPhone 6s plus the other day and she put a passcode on it and now she can't remember it. I can't access the phone on iTunes without doing the passcode either. What should we do?

Enter DFU mode - https://www.theiphonewiki.com/wiki/DFU_Mode

From there you can restore without needing a passcode.

If you have Find My iPhone enabled then you can log into www.icloud.com and remotely restore the device

You'd have to reset it.