Easiest way to do this on a site is to define the font in a style sheet. If you're not sure what a style sheet is, it's simply a text file saved with a .css extension.

See w3schools/css for more information and tutorials about style sheets.

Add this to your style sheet:

body {font-family: arial, helvetica, sans-serif;}

This line defines the font for the entire site. 3 fonts are listed in the example - arial, helvetica, and sans-serif.

Arial will be used if available, (this you should change to the font you'd prefer).
If it's not available then helvetica will be used, if that's not available, sans-serif is used, if that's not available then the default on the users computer is used.

You need also to put the following code in the header section of your html code:

<link rel="stylesheet" type="text/css" href="style.css">

Assuming that you've called the style sheet style.css

All the best - Duncan