I have a webpage, actually a blog, posted with Github Pages. It's a simple HTML&CSS page. Normally, I create new files with my new posts in them and upload these files to my repository. However, I want to create an admin panel. Especially in order to post easily, and manage my blog (like adding tags, comments etc). I don't know where to start or what to use. I know how to program in C & C#, so it's not a problem if I have to learn a new language. Any help would be appreciated.
You may be able to use a Headless CMS. These approaches normally are driven by git or some kind of API (you don't have to write any backend code) to add content to static sites such as yours. Although most of them work with markdown, so you may need some way to render the markdown into your HTML.
Headless CMS is normally used within Jamstack projects, so I'd suggest checking that out if that is something you're interested in.