Okay, first you need two little graphics to indicate to your user that they can resize the text.

Then you have the text. This works on the paragraph tag, so make sure your text is contained in paragraphs in the code.
Also, it works best if you size the font in pixels in your style sheet.

Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Sed ut turpis et metus mattis ultrices. Have you found out what your neighbour's funny building is for? Donec tempus. Maecenas tempus neque eu ligula. Donec sagittis pede id odio. Aenean arcu mi, convallis eget, pretium at, venenatis id, leo. Proin aliquam cursus dui. Quisque pellentesque felis ac magna. Ut egestas vehicula diam. Pellentesque vel eros. Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aliquam suscipit posuere turpis. Sed blandit. Nunc ornare faucibus quam. Integer aliquet gravida eros.