How do you interlock 2 pieces of plywood
A router with a straight bit or a table saw with a dado stack are the two best options for this task.

A dado stack will be more versatile if you have multiple widths of plywood that you want to work with:



A straight router bit isn't adjustable in width, but it will produce a cleaner bottomed cut. They're often available in sets for different sizes of plywood:



If you'd rather not spend the money on either of those, you can always use a table saw with a regular 1/8" kerf blade and make several passes. Then you can clean up the bottom of the groove with a chisel. I wouldn't recommend trying this with a circular saw, but if you do, you'll definitely need to make a straight cutting jig for it.