forgot my iphone passcode

If the iPhone is disabled, you cannot back it up.

For the OP, the instructions for restoring your iPhone are here. If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

You can remove the passcode by doing this

1. wait until it is no longer disabled ( thank you Chris)
2. connect your phone to your computer
3. encrypt the backups
4. back up the phone
5. put your phone into recovery mode
6. restore the phone
7. restore the backup
8. all passcodes are gone