There is a webbased game called TCG ONE, but I find it to be very difficult to interface with. PTCGO at the above URL is much more user friendly. The main difference between the two (other than PTCGO being a client) is that you can use whatever card you want on TCG ONE, whereas you have to acquire cards on PTCGO... but that's half the fun!
I have only been playing since March, and I have become totally obsessed with the game (although I don't play in real life, only online). I would also advise you to watch videos of common you tubers and go through the many deck lists and other advice you'll find here on pokebeach. If you have some spare change, there are websites where you can buy online codes for packs for usage on PTCGO. One thing I would advise though is to research various deck lists before you commit to buying. For example, I recently built a Mega Scizor deck. I compared the different deck lists from 7 different you tubers and took a compilation of those lists to make my list. You will also be able to find a number of deck lists here on pokebeach as well, but the advantage of watching the you tubers is that you actually get to see the decks play out, and that may influence your decision on certain cards.
I would also advise you to be a little wary of any advice that begins with "I think" or "I feel." There's a TON of subjectivity surrounding this game with very little data to back it up. I know I'm unusual, but I keep detailed statistics of every game I play, and I KNOW (not think or feel) that that has helped me improve my play as well as my decision making in deck construction.
Good luck and have fun! But be careful, you might get as addicted to this game as I am.