Have you tried checking the LFG section of roll20? Look for games that "welcome new players." If you find a game you're interested in, make sure to communicate to the GM(game master) of the game that you are brand new to gaming. Hopefully they will seize the opportunity to help teach you how to play their game. Some systems have a free "sub-set" of rules available online. ie D&D 5e, Pathfinder, Dungeon World, etc. Other's, you may have to purchase a book/pdf. There are mannnnnnnny more players than GM's, so you might have an "adventure" just trying to get into a game, but don't give up. Roll20Con is just around the corner as well which often creates a few more games than normal, which might help... Good luck.