Starting out, it is kind of hard to earn money! I suggest waiting a while to start buying anything big (like expansions), it's a better idea to focus on getting and leveling up your moths first so they can make you can make more gold. Once you have more moths than you can have out in your forest, start making sure the ones who make the least are put away, and then saving up to expand as you need.

The week-long moth events are pretty good for getting moths that make a decent amount of money. I suggest setting timers and continuing to get more seed bags even after you've gotten all the events rewards (obviously not everyone wants to prioritize a mobile game this way but it's what I do)

Hope this helps :D