Semprini said:
Where to start? There is much to learn. If you ask me, the best teacher specifically for microstakes exploitative play is Nathan blackrain79 Williams. Google him. He has tons of YouTube content. If you like some of that, check out his website.
I think "Hungry Horse Poker" content on Youtube, also "CarrotCorner" and "Live Poker guide" can be good source for Cashgamers, though "Hungry Horse Poker" and "Live Poker guide" are designed for Live Casino Cashgames some concepts are good for online too.
For Tournament play GTO4OMC may be a helpful youtube source.
Also most major Training sites offer free information (PokerCode, Raise your Edge, Jonathan Little, Upswing Poker and so on).
For starting out Charts you can use the free GTO-Wizard charts.
For advanced concepts there is always a paywall.