I'm working on a site to help students with ACT prep, and I want to have a page where I can post explanations to questions that people submit. I want to be able to put a few tags on each post so that site visitors can click on or search whatever's relevant for them in the archives ("semicolons", "geometry", etc.) and all the relevant posts will come up, blog style. I'm very new to this, though, and I don't know how to do it or even what to search - when I search for tags I keep getting SEO recommendations, and that doesn't seem like the right thing.
It might be the only way to make what you want happen with a static HTML site.
You could, by hand, create pages that you fill with links to all of the posts that fit a certain category or "tag". For example, you could make a page that has links to all of your posts concerning geometry. Lets call this your archive page for geometry.
Then, when you include tags in a post, you would make each tag link to it's corresponding archive page.
Virtually every blog that you see has a "back end" with a database that stores posts. When someone comes to your website and looks at a post, that posts data is inserted into a template and displayed to the user. You do not have to re-write the entire web page every time. Thing like the header, sidebar, footer, main page background etc are all in a template.
Having a database also lets you search the database and return relevant results. And a blog with a back end will typically let you write rules (or have them already written) that say, when you add a "tag" to a post, a link to that post should be automatically added to an archive page etc.
As far as I can tell you don't have database, so you'll just be linking static HTML pages. That means that every time you make a new post, you'll have to add a link to all of it's relevant archive pages by hand. Maybe you don't mind that now, but eventually it will be a nightmare to maintain.
I would strongly encourage you to look into a blogging platform like Wordpress to make your site. It will be more complicated to learn at first, but technology that's meant to do what you want it to do will ultimately be easier to use and maintain than technology that's simply meant to mark up a page.