Don't be too discouraged by this. There is a lot to learn before you'll be able to go deep into arena.Welp, lost all my Arena games again.
One more time:
Warlock
Warrior
Druid
Lol.
So, Druid or Warrior?
Make sure you try to get every class to lvl 10 or even 20. That way you get a chance to see what they have to offer.
Arena will be tough until you know when board clears come up for each class and test for them before over extending yourself. Learning about each classes removal spells and experiencing their best cards in the worst situations. This only comes from experience.
There are some cards you'll likely only see in arena. Browse all the cards in the game also. Go to Crafting mode and take a look at what's there. It can be helpful to just have some familiarity with it. The first time I saw an Ancient Watcher was in arena and when buffed it into a taunt it blew my mind.