How should I add more energy to my deck?

It depends on your deck, but generally running more draw support to get you into more energy is better than just running a ton of energies.

I'm pretty sure the only way to get coins besides playing games is to redeem codes, or I guess using gems to skip ahead on the battle pass. You get coins even if you lose though, so you might as well just play on the casual ladder for a bit.

For a typical deck, aim for about 12 Energy (all the same kind), and then 8-10 Supporters with card draw effects (Iono, Professor's Research, and Colress's Experiment are all fantastic options). Do this and you usually won't run out of Energy to play each turn.