How to make coin in nulledbb

By coin, I assume you mean credits. Here are some direct quotes from the Help Center.

How are credits earned?
Credits are mainly earned by contributing to the community - either by creating threads or posts.
They can also be won by participating in the raffle or inside the Arcade.

Each time someone unlocks on of your posted resources, you will receive 8 credits.

The gained amount for contributing differs:
Post: 1 Credit
Thread: 2 Credits
When someone unlocks your resources: 8 Credits