The vast majority of bikes will put the serial number under the bottom bracket. You’ll need to flip the bike over to look for it.
The serial number will be embossed in the frame in most cases, but there may be a sticker as well. You might want a flashlight to help you read the embossment.
What if it’s not there?
There are less common but still frequently used areas to check for a serial number. They are:
A: On the down tube just above the cranks
B: On the rear stays, which hold your rear wheel
C: On top of the crank
D: Under your top tube
E: On the headset
And if you still can find one, don't fret. Some super custom bikes just don't have one. You may type in NONE in the serial number field and we won't bother you about it.
This was one contributing factor as to why we developed our shields. You may see about ordering one of those here.