If you're a seasoned closed beta player, prepare to welcome a lot of new opponents. If you're brand new to the game, have a look at the tutorial, and maybe peruse WoW Insider's Hearthstone guides, articles and offerings.
Fear not, you won't be crushed by the overgeared. Hearthstone's matchmaking will put your against opponents of equal skill so that everyone can enjoy themselves. Have fun climbing the ladder, crafting cards, and stepping into the Arena!