Merits are given by the other members of the forum for the good quality posts you do. For Ranking you need both activities and Merits. Check the Ranking system below -
Brand New: 0 posts - 0 merit
Newbie: 1-29 activity - 0 merit
Jr. Member: 30-59 activity - 1 merit
Member: 60-119 activity - 10 merits
Full Member: 120-239 activity - 100 merits
Sr. Member: 240-479 activity - 250 merits
Hero Member: 480 activity - 500 merits
Legendary: Somewhere between 775 and 1030 activity -1000 merits
Activities and Merits are different and are not related to each other. Activities will be based on the number of posts you do. You can get 14 activities every fortnightly period. Activity is calculated using this formula -
activity = min(time * 14, posts)
Don't think about merits. You will get merits if you do some quality posts. Post some useful topics, spend some time to contribute on the forum, help the users by answering their queries. This will automatically get you merits.