Inserting an increasing font size icon

"New to forum, just have a question for you really smart folks.... How do you insert an icon (usually different sizes of of the letter "A") on a web page (usually at the top) so that the user can select the size of font that they will be able to see better? I know you can just "CTRL +/-" and do this with IE but I'm working with older folks that need to be able to click something." ~Harris2133