Be careful of youtube videos. As SB said about the doctype, some of those videos are either incomplete or just flat out wrong. You should go to some respected coding sites for good tutotials such as w3schools or w3c.org
There are also some very good books out there relatively inexpensive that has step by step tutorials along with downloadable excercise files to work through. HTML and XHTML New perspectives 5th edition is a very good book.