To start out, there are a few tags you should know of. They are listed below with a short explaination of what each tag is used for.

• <div> : the div tag (division) is a block level tag used to define a section in a web page. This tag is most often used for the layout of a page.
• <span> : the span tag is an inline element used to break off a section of text. without stylesheets, this tag doesn't look like anything.
• <i>, <u>, <b> : these, among other tags, are used to change the presentation of text. the examples are italic, underline, and bold.
• <h1> - <h6> : These tags are used for headings, <h1> being the biggest.
• <a> : the anchor tag. Used for links and other things.

There are many more tags, but those are the tags that will be used most often. For a complete list see: http://www.w3schools.com/tags/default.asp