Just enter the tags that are most relevant to your post. (up to 5, all lower case, separated by comma)
A full list is available here:
https://steemit.com/tags

You may also want to look at the Steemit FAQ page (available from the menu pulldown at the top right corner, or here:
https://steemit.com/faq.html

It is also considered bad form to use the introduceyourself tag beyond your initial introduction, and could result in penalties....

Enjoy