You don't need to know any criteria before you earn merit, because theirs people who newly registered but the first post or second post they make and they earn merit, why someone who has been here for awhile has not earn any. But follow this process it can help you to earn merit faster.

• Read and understand posts before you respond

• Make a research of something that has not  been written and post in forum before

• Desist from low value post, making a quality post will give you high chance of earning merit

• Ensure your subject in any of your thread and the composition is in accordance

• Make sure your spelling and the English you use is properly arranged or  polished 

• Make sure you post in your locals, because you have more advantages of earning a merit in your local board than English boards