This is a good way to learn the basics:
https://www.pokemon.com/us/pokemon-tcg/play-online/
If you're concerned about card value, check online, troll and toad is a good site to get a relative sense of the monetary value of the cards, which generally corresponds to how useful they are in the game. Most modern cards are equally worthless, however, so you should just let your kid do his thing and collect the cards he likes.
My kid is finished with his Pokemon phase; now we have boatloads of cards that surely will never be used again. It gets out of hand in a hurry.