The Steemit algorithm looks for 5 tags max. The first tag should be a genre tag most relevant to the post. Each following tag should be narrow enough to be descriptive, but broad enough to have search appeal. Take a look at the top trending tags to get a feel for which popular genre tag best fits your post.
https://steemit.com/tags
Abuse of the tagging system is monitored a group steemcleaners.org. I believe the Steemit username is: steemcleaners. You can also appeal to steemcleaners.org if you feel that your content has been plagiarized.