Yeah, as already stated, early on just worry about making your top cards as good as you can. Once you get a little higher up in the tiers you'll need to worry about balance, but for now just leveling up is the name of the game.
Make sure you properly pro your cards. You'll want to get both cards fully leveled up and all matches played on both singles before you combine them. This will make a SIGNIFICANT difference as opposed to just throwing two cards together without prepping them.
Always claim your free packs, including the bonus one for watching an ad. For one thing, those cards will probably help out your beginning deck a whole lot, and also those two credits will add up over time. They don't seem like much now, but between that and the daily login bonus packs (make sure to at least log in once a day to get those, they're worth it) your credits will start to go up quickly.
