I was thinking that becoming a follower will let me know when there are new questions about the tags I'm interested in. However I can't figure out how to become a follower or if becoming one will accomplish what I want.
You can 'favorite' a tag, there's an item in the sidebar for it:
Questions with a tag that you have favorited show up in a slightly different color on the home page and question lists, allowing you to find them more easily:
However, it sounds like what you want is to be able to see new questions in that tag. In that case, just search for [tag] - I.e. for [gravity]. You can combine them by searching for
[tag1] OR [tag2] - like this for [gravity] or [planets]. This way you can have a page that you visit to see questions that are in tags you've expressed interest in.