Welcome to the Pokemon TCGO discussion thread!
You can download the game here: http://www.pokemon.com/us/pokemon-tcg/play-online/download/
Here are few useful things for in-game:
Gentleman's Agreement: https://www.reddit.com/r/ptcgo/comments/5d1oq3/card_value_guide_updated_info_on_gentlemens/
The Gentleman's Agreement is basically a way to accumulate a lot of "Versus Points" very quickly and not have to waste hours every day, just barely meeting the goal of 2,000 for the final prize in the Versus Arena.
Card Value Guide: http://www.ou7c4st.com/how-to-use-card-value-guide.html
This is the most popular card value guide, when you get to the point where you have accumulated a decent amount of tradable packs and/or cards and you are ready to build your first actual deck, then I highly suggest looking over it so that you aren't getting ripped off in trade-deals.
In-Game Free Codes:
plasmastorm (power relay deck)
boundariescrossed (Power relay deck)
dragonsexalted (Pokemon EX Hat)
darkexplorers (Pokemon EX Hat)
Some of these may not work anymore, I can't tell since I've already claimed them, but feel free to go ahead and try them, just enter them into the "Redeem Codes" section in Pokemon TCGO.
If you need help with anything, just post below and one of the more experienced players will likely be glad to help in any way he or she can. ^.^