Couple of noob questions

Hi, i recently started playing this game and have few questions. My max stage is only 6.2k atm.

- What is a Heart of Gold? Like whats the source for gold from that?

- Which is a good gold source at the early stages of the game?

- With the clan ship build is it better to use slash primary effect (with clan ship slash equipped) or primary sword effect with (all damage sword equipped? Or a different combination?

- What are some good sets to craft to start with?