What should I do if I locked myself out of my phone?

you have specified the model of the phone, but anyways most samsung can be restored the to the factory settings;

Turn off the device.

Press and hold the following three buttons at the same time:

Volume Up key

Home key

Power key

When the phone vibrates, let go of the Power key but continue to press and hold the Volume Up key and the Home key.

When the Android System Recovery screen appears, release the Volume Up and Home keys.

Press the Volume Down key to highlight 'wipe cache partition.'

Press the Power key to select and wipe the cache.

With 'reboot system now' highlighted, press the Power key to restart the device.

once its restored you can sign in with your google account and retrieve your last backup.