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Find a topic that interests you. One you can really go into detail with. Or if you want to push yourself you can always research a new topic that you want to learn and put this information into a post. Putting everything you've researched into a post is also a good way of improving your own knowledge.

interesting ...thanks buddy but i still dont know how it works i mean what is the criteria on which they gave the posts or comment the bucks? you know that?

It's a combination of things. Effort, content and luck. Just need to make sure your post is more likely to get noticed.