You have to spend coins to unlock new stages, and gems in order to get more energy, and earning coins is done by trading in artifacts that you play for coins. The more times that you play a particular stage, the more of that artifact that you will earn. Make sure to also keep track of the quests that you’re accomplishing, because you’ll earn LOTs of gems and some coins when you complete them.
The main way to get coins, of course, is to play as many rounds as possible so that you can get artifacts, and then trade them in for coins. Replay rounds and memorize the location of objects each time to maximize the quantity of relics that you earn every time that you play, because the better you do on a round, the more relics you will earn. Build up a large cache of relics and there will be trades that open up for them, since trades refresh every few hours and every time that you actually make a trade.
Keep an eye on the quests, as well, and complete them in the right amount of time, because they’re the best way to earn LOTS of gems, and some coins as well. If you don’t complete a quest within the allotted amount of time you won’t earn the gems and the coins, but another quest is sure to pop up right afterwards.
To get more shovels without having to pay for them, play stages. Each time that you play a stage the shovel’s bar will increase, and when it fills up it resets and you get three free shovels. The higher your score, the more the bar fills, so it is recommended to play the same stages over and over so that you memorize where every object is, and thus, can tap them more quickly, allowing you to refill the shovel gauge quicker.
Later on in the game you can unlock new relics for old stages. Tap on the stage that you want to unlock new relics for, and you can pay a certain amount of coins for the unlock. Then, go to that stage and play it, and you will be able to find the new relic, and earn it for your cache of artifacts after you beat the stage. It’s recommended to wait until later in the game to do this, though, when you already have a large number of coins to play with.