If you just started or low level you might be under impression that there is a plenty of gold to go around. This simply is not true.
Thing is that at lower level your cards cost just a few (hundred) gold to upgrade and you begin with nice amount of gold. Also gold gain on low level is the same as the gold gain on higher levels, which deepens this misconception. Point being that either way you are limited with daily gold limit (10k gold, but to be checked might be way off).
As you level up you will notice that card upgrades might be 10k gold even which is whole day playing for one card upgrade.
Now this is not a guide how to spend your gold, just keep in mind that you will not have enough to lvl all cards so it might be better to go with neutral cards that you use in all decks, after that pick a class that you will play most and go with that.
Also take into consideration that not all upgrades give you same resistance bonus, note that 1st upgrade for epic card is 10k gold but will give you 1000 resistance, this is a great imo, in compare to common card upgrade that will give you (for example) 60 resistance for 2500 gold.
Last thing you get gold playing no matter if you win or lose, take advantage of that!