Unfortunately, there aren’t exactly any ways to test decks without having to do a little work (you have to have the cards to play TCGO). However, you can still build decklists there, and export them as a text file.
As far as good resources/YouTubers go, I’d personally recommend The Pokemon Evolutionaries or Yellowswellow, just because they were the place I got most of my info from when I started out. A lot of other strategy sites exist, but for a lot of them, a premium subscription is required. If you are looking for good decks to play, start by finding YouTubers that play a lot, and have a decent variety. Twitch is also a good option if you can find some good streamers.
A bit of the side question though, do you already have cards to work with?
That aside, if you have more questions, feel free to ask! I’m still a bit of a beginner, and the decks I play aren’t exactly the best, but I do at least have a decent idea as to what I am doing.