If it has an extended oil-fill tube (4"-6") it probably doesn't have a drain plug. Either way, just tip it up and drain it out of the fill tube.
This begs the question of how can you have a mower and not know how to drain the oil from it - the answer must be 'no oil changes'.

That model has the perpetual oil change feature. Just top it off as the oil burns off and leaks over the years.

I'm sure that the junkyard that mower is likely headed for would also appreciate it if you drained gasoline form the tank and carb.

donate the mower to the local high school for small engine class, or give it to a repair store for parts, or just leave the mower in your front yard at dusk and will be gone the next day. alot of people do side work striping appliances and lawn mowers for metal to scrap for money

Better yet, and faster---set it out front with a "for sale" sign on it, with a price of $50, and it'll be gone quicker than that! Be sure it sits close to the curb! Some folks just can't pass up a good bargain, especially after dark! ;o)

Check out freecycle.org. There are local versions almost everywhere and you can post the mower to give away. I gave away an old lawn mower engine to a guy who likes to tinker. He came and picked it up. I also gave away an old non-working computer to a guy who wanted it for parts.

I tried that Free-cycle thing. I had an old item i would give away if they came and picked it up. they came to look, but didn't want it. They asked if i had another item like it. I told them it wasn't free. So, they complained to Free-cycle, and my name is now "Mud"!
Oh, well, i got along with out Freecycle, so i guess i'll make it without them ,still.