There is a store next to you in the room you spawn. I didn't give myself enough time to make much art for the game so it's just a white rectangle that says "Store" above it hahaha.
You get money by pressing "E" on the stones with gold nuggets in them in the caves. You can then go back to the store, press E on that too and a menu will pop up where you can buy upgrades.
Thanks for playing!