Use this forum and basic concepts you've read in books. Once you've reached $25nl you can think about joining a training site. You can join earlier, but I can't really see anyone getting much out of it considering the play at $2nl - $10nl is beyond terrible that any patient monkey can beat it. In my experience, $25nl is where you'll start running into a few more regs as well as players dropping down from higher levels. Don't get me wrong, the play there is still terribad overall but this is where you actually have to start thinking about your decisions to grind out a decent win rate and not just autopilot for 5bb/100.

Go with Grinder School, at least to start out imo, because it's light on the wallet. They have some pretty good series and lectures that will improve your win rate overnight.