If you just mean how to set things up so people can change the size of the font itself just use measurements like '%' or 'em' which aren't fixed and your visitors will be able to make the font size larger or smaller as they see fit.
If you mean you want the area containint the text to expand and contract with the resolution of the browser then it's a similar concept in that you will want to use % for many of your widths, but it's a little more complicated. The idea is a fluid layout and it can take some practice to get right.
