Hello!
Good questions. Try to use all the 5 tags allowed. Some of the most popular are photography, art, nature, life etc. You can check the stats for the most popular tags by following @arcange. There are various challenges ongoing on Steemit. I usually mention the ones I know about in my posts.
For more tips on how you can increase your visibility, you can check one of my old post: https://steemit.com/steemit/@alinabarbu/a-letter-to-newbies-on-steemit
Hope this helps! Feel free to reach out:)