Gemp is free as a bird, my fellow! You are given all cards to play and build decks. (Even some Fan Made formats, like the Second Edition and the Hobbit Draft Game!)
There is some vitual currency to play leagues and compete, but has no relation with anything in the real world (it isn't bought, you start with 200 gold pieces, and each week you get 50 more). You can purchase cards in the Merchant but there's no need at all to do so anymore. The Leagues that used cards of your "My Cards" collection are no longer done (and won't return). So just build and play, my fellow!
Best format to start? Fellowship Block. Fewer cards, combos and rules. Ask for help in the Game Hall chat (the place where you start after login) if you need, and ask your opponent for patience and say you're a beginner in Gemp's interface. Be polite, and enjoy!