How can I unlock my iPhone?

Plug it into your computer running the current version of iTunes.

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled

In order to unlock your iPhone, you will need a computer (either a Mac or PC), hosting iTunes. Start up iTunes, and then, using a USB-to-Lightning cable, connect the cable between your iPhone and the computer's USB port.

From there, you can use the instructions that KiltedTim has provided you.