For plywood you want a downcut bit.
You should be able to pick up a 1/4" downcut bit in the router bit section of your local hardware store. (Both Lowel’s and Home Depot stock them)
The downcut bit pushes down as it cuts, instead of lifting like a traditional bit.
This keeps the bit from tearing up the edges of the plywood as it cuts. It also keeps form lifting the center of a large edge clamped sheet up as it it cuts.
Here is an example of an array of larger holes in plywood I made with a 1/4" downcut bit:
(click the [image] link)