First time credit.

Just curious as new here. What’s the process to request credits and then use them, going forward?

There are no "credits".
The guy members can earn or buy premium access which allows access to other forums.

I mean credits after posting encounter reports?

Once an Encounter report is moderated, if determined acceptable, the contributing member receives 6 weeks of premium access. It's mostly automated.
Read the heading paragraph at the top of the Encounter Reports forum.
Also consider reading the thread "KosherCowboy's Sample Reviews" in the same forum.

Once a review is moderated, a PM (private message) will be sent to the contributing member about premium access.
However, if by chance a review is rejected for PA, the contributing member should send a PM to that specific Moderator with corrective information in that PM if reconsideration of PA is desired. Note that some rejection issues may not be able to be satisfied.