On most bikes, the serial number will be under the bottom bracket. You'll need to flip the bike over to look for it.
In most cases, the serial number will be embossed in the frame, but there may be an easier to read sticker as well.
You might want a flashlight to help you read the embossement.
What if it's not there?
Other places to check are:
The downtube, just above the cranks
The rear stays that hold your rear wheel
The top of the crank
Under your top tube
What if I can't find a serial number?
Unfortunately, some bikes simply don't have serial numbers, so type None into the serial number field and keep on going, and make sure you enter as much information as you possibly can about your bike.