Here is some bold text, some italicized text, and some underlined text. Of course, any of these effects can be combined.
You can also add subscripts such as H2O and superscripts such as E = MC2.
Many characters in html also have a numeric "code". This is particularly useful for characters that html interprets as part of tags, such as < and >, or characters that don't exist on the keyboard such as ∞. This can be done by using the & character followed by the character abbreviation, followed by a ;. A complete list of such codes can be found in any html reference.
You can also use properties of the FONT tag to more closely control the size, color, or font of the text, or all of the above.
However, a better way to control the size is probably relative measures, using + or -. Here is some larger text and some smaller text.