To unlock your Apple Watch SE, first, you have to wake up your device. To do that simply raise your wrist or tap the screen. Once you’ve done it your Watch will be awake for a couple of seconds (depending on a preset time).
Now you can unlock your Watch by simply press the Crown button (on the right side of your device). You should be let into the home screen if your device is not protected by a passcode.
If your Apple Watch is protected by a passcode, you have to enter your passcode. If it’s correct, you should be let into the home screen.