My first week playing Splinterlands

Hey everyone,

My name is Trix and I recently started playing Splinterlands. And I am surprised by the quality of the game.

In fact, I am playing like crazy, I really enjoy it, and I have achieved the bronze league so far.
I think that if I continue playing like this I"m gonna be addicted to it.

Anyway.
I have a lot of questions about the game itself, if somebody could help me out I would really appreciate it.

First of all I would like to know what we can do with the dark energy crystal?

Is there any other marketplaces where we can buy and sell cards?

How can I participate in the Splinterlands contest?

How can I transfer dark energy into cryptocurrency (Hive and bitcoin)?

If somebody could answer in the comment section it will be really nice ;)