Level the mage up to 20(?) so you can do Tavern Brawls for the weekly free pack.
Definitely try arena (check a few guides / tier lists first) as it'll let you play with lots of cards.
However most of the best cards are still in the core set if you decide to buy packs.
If you want to dive into constructed on a budget, look for aggressive versions of Zoo warlock or Face hunter, they're cheap and effective decks. control decks are pretty expensive when you're first starting out.
Definitely try arena (check a few guides / tier lists first) as it'll let you play with lots of cards.
However most of the best cards are still in the core set if you decide to buy packs.
If you want to dive into constructed on a budget, look for aggressive versions of Zoo warlock or Face hunter, they're cheap and effective decks. control decks are pretty expensive when you're first starting out.