Which post is worthy of reward?
Helpful and on-topic. There is no rocket-science in this. You don't need to know a formula to become worthy of Merits, but your posts should contain something that should be helpful for the community, or an individual.
What it have to consist of?
Correct grammar, meaningful words, logical sentences, a good construction. Overall, it should deliver some information that has not already been delivered.
Which words posts should include?
There are no specific words someone needs to include in a post to make it meritorious or constructive. The words depend on the situation and content you are talking about.