You will be able to access the device using your Google account details.
You will have 5 chances to enter in your pattern or password in correctly, before the phone locks you out for 30 seconds.
On this screen, you should see an option called "forgot your password".
Tap 'Forgot Password', and you'll be prompted to enter the Google account details associated with the device.
Once you enter the details, you'll be prompted to create another security code.
Please change this to something you will remember.
If you wish, you may remove it once you have done so.