Tags are for specific areas. People, Events or things that are referred to in your posts. For example “2019 Australian Open” or “Joe Bloggs”. For events, make sure you add the year.
After you type the first few letter of a tag, it will pop up with suggestions and that is the easiest way to find the tag you are looking for. If nothing pops up then that tag has not been entered before and so you can just continue typing it out.