There's Several ways to earn Merit. Forum activity automatically increases Merit. Activity is defined as posting on the forum (keep in mind that chat box does not increase Merit).

On forums:
New topic: 3 Merit
Reply to topic: 1 Merit
New Topic on Suggestions Forum: 5 Merit
New Topic on Events Forum: 15 Merit.
(Keep in mind that posting for the sole purpose of earning Merit is strictly prohibited by code of conduct)

Other Ways to Earn Merit:
Attending Events: 15 Merit
Assisting with Recruit training: 20 Merit
Running Recruit Training: 30 Merit
Having Submitted suggestion accepted: 15 Merit
Other Notable accomplishments: 5-30 Merit

Earning Reputation can only be done in 2 ways.
Members can give positive or negative votes.
Positive Vote: 1 Rep.
Negative Vote: -1 Rep.
Thanks Button (members you give rep. to may click the thanks button to provide you with bonus rep.): 3 Rep.