First you should know about basic tags like: <html>, <head>, <meta>, <link>, <script>, <body>, <div>, <h1>, <p>, <ul>, <li> You can get help from http://w3schools.com/ All the best.
w3schools.com is great place to start with HTML . They have also very nice tutorials about CSS and JavaScript.
Try tizag.com. They provide various tutorials in a very user-friendly manner. Most tags are very important but you'll probably want to learn how to create links, want to use "breaks", create headlines with h1, h2, h3, tags, etc. and start new paragraphs above all else. Those are the most basic tags. You may also want to know how to create meta tag elements.
I recommend w3schools. Unlike everyone else. I also recommend the p tag. And maybe hte div tag. Just worry about those