What kind of decks have you enjoyed playing or watching these past few months? Aggro, control, tempo, midrange?
Start with that and then choose a class that you think best represents your playstyle. After that you can start building your deck, analyzing synergies, mulligans and strategies.
People in this site are ready to answer all of your questions (dont mind the trolls, these guys are everywhere). I'm more than happy to help, I play on EU Aizen#2772.