What is the best way to learn to play dnd on roll20?

I play dnd with my friends and I know a little about how to play but as we have never played "professionally", I guess you would call it, so we don't know the rules and how to play perfectly. So my question being what would be the best way to learn here on the site.  Ps: I have read the basic player rules and the DM rules but I'm not completely versed in them so I apologies if I don't understand something you say 

Very few folks have played "professionally". It is still very much a hobby. As such, the best way to learn to play is with folks who already have some experience. The LFG forum and tool have many games that welcome newcomers. Roll20 has a Twitch channel that has many games played openly for any to watch and learn both a particular game system, but by extension, the Roll20 interface. Just remember that the only real way to play an RPG "wrong" is to not have fun. In the case of D&D in particular, a good DM will describe the situation, ask you what you want to do, then either resolve the situation or ask you for a specific roll in order to resolve it. Knowledge of the rules comes along the way. After a few sessions, you'll no longer be asking "What die do I roll?" :)