The frontend is probably the most important part of your application. It is the way people actually interact with your app. If your interface sucks, people are not going to enjoy using your project.
Since your project will need to interact with your backend, I would reccomend using a framework, rather than plain HTML and CSS. Some popular frameworks are React and Vue for JavaScript.
The backend is where a lot of the logic of your application will go. For this guide it will also be serving your frontend, but it is also possible and sometimes recommended to serve your frontend separately. The backend is the connection between the frontend and the database.
There are lots of backend frameworks to choose from. Some popular ones are Express for JavaScript and Django, Flask, or FastAPI for Python. It is also recommended to document your API using something like Swagger (OpenAPI).
The database is where all of your applications data is stored. There are many options like SQLite, PostgreSQL, MongoDB, and more. SQLite will be the easiest option since it is just a file and you don't have to host a server for your database in addition to your backend. This guide will show you how to set up SQLite or PostgreSQL with your app.