MDN Web Docs
MDN Web Docs is a resource created by Mozilla, the company that manages the Firefox web browser. It provides a huge selection of tutorials and resources for every part of web development. Most of it will not be relevant to you yet, but they either provide or endorse a number of resources you may find useful. For example:
w3schools
w3schools is a resource aimed at web developers that covers just about every HTML concept or element one might come across. Most concepts include brief explanations and demos. Given the quantity of concepts, though, you will probably find it most useful as reference, similar to the MDN Web Docs resources above.
Stack Overflow
Stack Overflow is a question-and-answer platform that allows programmers to ask questions, which other programmers then answer. It is used regularly by programmers of every experience level, so it can be intimidating at first, but it can be invaluable when you run into problems or get stuck.
Anybody can ask questions on Stack Overflow, but for beginners, it's most useful as a record of questions others have already asked. Almost any problem you experience or concept you find confusing early on has already had a question asked about it. If you have trouble with anything, after all, you can rest assured that you are not the only one.