I circled the deck plug in red on the attached picture to help show where it should be.
Newer ones are plastic and can be twisted out easily by hand to open.
Older versions are metal, and you need a screwdriver or a coin or similar to unscrew them.
Unless you are racing, a far better place to install a drain plug is in the transom on either side of the gudgeon plate. You can buy a screw-in drain plug with a metal or plastic flange like comes standard on all Lasers.
In my opinion the original metal drainplug on all pre-1990(?) Sunfish is practically useless and requires you to either stand there holding the boat up on its side while you wait for water to drain or you need a building or vertical surface to lean the hull up against.
In salt water, the corrosion will seal the threads permanently unless lubricated frequently. It was never a good drain plug or a good location.
On each of the hundreds of Sunfish I refurbished back on Cape Cod during the 80s & 90s this was the very first thing I would do to each hull, (the 2nd was to install a Harken swivel cam cleat on the front lip of the cockpit).
My boat doesn't leak, so I like having the plug on the deck. I never have to worry about it having come loose as on the transom and having the boat fill up if I don't notice it is loose. That said, the plastic plugs are a heck of a lot better than the metal ones, for the reasons that Alan states.