After finishing those tutorial games, the game will automatically select a region for you based on your most deck / cards from the default ones. Then, you will start earning EXP and rewards mostly specific to that region, but not only. Because the Wildcards that you get as a reward can be used for any type of card / region, then it's good to try and do as many missions which give Wildcards as possible. Every region will have 20 missions in total, and since there are 6 regions you will have more than enough missions to go through!
You can change the region that you play for when you head on to the Rewards tab and there you will see like a "refresh" button which will let you change the region. Make sure that you try to play and claim at least the first 7 missions because from that Champion Capsule you'll be able to claim 3 Rare, 1 Epic and 1 Champion cards! The rewards get better the more you complete and play for 1 region, so if you don't mind farming quite a bit, then this is a great way to get good loot and cards!
