Depending on the manufacturer of your laptop the model number can be found either on the plastic frame (also called front cover) around the screen or on the panel right under the screen. Some manufacturers (e.g. Toshiba, Gateway, e-Machines) place the model number on the sticker on the bottom of the laptop.
Please, note, that Sony usually puts 2 different model numbers on their laptops. One is on the lower corner of the plastic frame around the screen or on the power button panel under the screen. The other one is on the sticker on the bottom of laptop. If you can't find your exact model number, PLEASE, DON'T PANIC. Simply Fill in the form to the best of your knowledge.