I have an older 20hp riding lawnmower that someone else operated last year, thus I know nothing about. I can't get it to start, not sure I am even doing everything I need to to get it started. Could you give me some basic guidelines to get started and maintenance I should perform?

Hello Ruthie, W/D here.
Push the throttle lever 100% forward, to the choke position. This will cause more gasoline to be pulled into the carburetor, making the engine easier to start. As soon as the engine starts, move the lever back towards you, to about 75%, and the engine will smooth out. Adjust the throttle from there to cut grass.
Oil in your engine is it's life blood, and should be changed after about 25 hours of operation. When you get your free manual (below), it may say different, so use 25 hours as a guideline for now. If you don't know when the oil was last changed, change it now. It is some of the cheapest insurance that you can buy. If your engine is equipped with a filter, change it at the same time. If your axles are equipped with grease fittings, by all means give them a shot of grease. Same goes for the spindles that turn the blades. These can be seen under the mower, on the top of the mower deck. You should look at your belts from time to time. Belts stretch out, and they can crack, making them less efficient, or cause them to slip, and have to be replaced from time to time.
You can get a free downloadable manual from searspartsdirect.com. You will need to enter your make and model number, and this site will give you parts lists with prices, exploded diagrams, and (usually) links to manuals.
Here's hoping that this helps get you mowing, and to maintain your mower. The throttle position is the most important part of getting started, but check the oil first. The grease fittings can be checked at your leisure, unless you start hearing the fittings making noise.
Best regards, --W/D--