There are plenty of documentation about this on Google.
Just use:
<!-- **normal** (iPhone/iPod **non retina** display) -->
<link rel="apple-touch-icon" href="apple-touch-icon.png"/>
<!-- **normal** (iPhone/iPod **retina** display) -->
<link rel="apple-touch-icon" href="apple-touch-icon-72.png" sizes="72x72"/>
<!-- and **iPad** version -->
<link rel="apple-touch-icon" href="apple-touch-icon-114.png" sizes="114x114"/>
As an additional info, you can have the icon without the gloss effect if you use apple-touch-icon-precomposed
instead of apple-touch-icon
.
and there are several tags you can use in order to have all features of Safari Mobile like splash screen, etc
also, give this Mobile Tuts article a read and you will know almost everything you need to have a brilliant web app