Are you going to invest money or just time into the PTCGO? Sorry to answer questions with another question, but it makes a huge difference on what you need to learn.
If you're going to sink some cash into the PTCGO so that you can get booster packs to trade for what you need, then you'll need to know the current metagame for the four PTCGO formats (Theme, Standard, Expanded, Legacy) right away. If you're going to try to play for free like myself, then what you need to focus on is the most efficient way to grind in game rewards from the PTCGO. In both cases you'll need to learn how to best trade because you cannot directly buy most important cards on the PTCGO; you can grind for booster packs to trade, you can buy firsthand product to get its PTCGO Redemption Codes, or you can find a trusted website to buy just the codes themselves.
If you're wondering why this might mean you don't have to worry about the Standard/Expanded/Legacy metagames yet, the short version is even if you plan on investing, I recommend starting with the Theme Deck mode just to get used to how things work with regards to in game rewards, tournaments, etc. on the PTCGO. I am running short of time now, so let me know if you want more help in this area, or are only worried about the online metagame. The online metagame is pretty similar to the IRL metagame, so the main thing to do is just google for results from recent tournaments. I know a fan run site that does a good job of giving you who won and with what, but I forget the linking rules for this site
and it just gives deck names/general descriptions and not lists.
Fortunately pokemon.com posts the Top 8 decks for many events, and are usually among the Google search results, like
this.