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I don know, what is your expertise. You should comment on those post which is well know to you. Firstly you read a comment and try to understand the main subject of the post. Then you write a good comment ( two or three line) on that post. if Author like your comments then you will be awarded by his upvot.

For your information you can go to this link:
https://steemit.com/steemit/@doctalk/common-mistakes-in-steemit