My motorcycle has been in storage for about 20 yrs. What should I do before trying to start it up again?

I think that is a no brainer. You should immediately truck it over to a motorcycle shop and let a professional peruse it and start it up after determining if that can be done right away. After sitting for 20 years, there wouldn't be a gasket that would be any good anywhere in it.

Tick

Absolutely take it to a bike shop and have them go over it. It may or may not need gaskets or the engine could be seized. They will be better equipped to deal with it.

Do you want to get it going as a project or as a going concern? I could agree with the other two answers unless you want to have a go just for the hell of it and to learn. In which case... take out the plugs, change the oil, disconnect the battery and put it on charge. I hope it was stored without fuel? Take off the alternator cover and with a suitable sized socket, turn the engine, very gently, by hand and in the right direction. Slightest resistance and you have a seized engine. Should that go OK, give it about a dozen turns, just to get some lubrication over all the engine parts. Put the altenator cover back on, reconnect the battery and presuming it has electric start, try turning over a few times. If not try the kick start a few times. Now put the plug (s) in and see how it goes, you could even take pressure readings at this point to ensure you have a working sealed combustion chamber. Generally spray something like WD40 or a general ptfe spray over choke cable, throttle cables etc and work those until you're happy. Now you have two choices, either put some fuel in the tank, or connect an old washing up bottle with some fuel in it, to the fuel line instead of the tank, that way you are only using 1/4 Litre or so. Now try turning it over again with choke and see how it goes - gently with no revving or every seal you have will go bang. If it works and the engine runs OK, we go to step two !
If it doesn't you either get the books out, ask again or revert to plan alpha - the shop.
Good luck, let me know how you get on. By the way, what bike is it??