Style sheets should be used if you use a certain text decoration over and over, and it just becomes too much to type out the font tag every time you want to change commonly used styles. Style sheets go inbetween the end title and the beginning body tag. Start the style sheets with the <STYLE> tag. This tag has no attributes.
First you must choose a font family. Type something like "H1" or "P" without the tags around them. That way, whenever you set type something inbetween these beginning and end tags, it will styled by the style sheet. After the "H1" type an opening "{". Then use the attribute "font-family." Attributes in CSS are seperated by a comma, not just a space, and use a colon instead of equal sign. After "font-family" type ":'YOURFONTHERE'" where YOURFONTHERE is in double quotes. Only the font-family attribute's value can be in quotes in CSS. No other value may be in quotes. Then end the CSS tag with an end curly-bracket "}".
Now, you can choose to apply bold or italic text. Use the font-style attribute. Then type italic. This will obviously create italic text. If you wish to create bold text, use the font-weight attribute with the value being "bold." For underlined text, use the text-decoration attribute with the value being "underline." Now you may want to set the font size. This is done using the font-size attribute. Value value may be "20pt" or "30px" for points or pixels.
Now it's time to set the font color. This is done using the color attribute. The value for the color attribute is a hex code.
To change the text's background, use the background attribute. The value for this attribute is a hex code.
To align text, use the text-align attribute with the value being the same as it would be if you were using the div tag.
That's all I know on CSS. If you wish to learn more, I urge you to take a look at the tutorial I referenced you to. CSS can be very useful if you can use it better than I can.