To do a hammer-on a note is picked once, then the same string is fretted to catch the second desired note without picking it again. Hammer-ons always result n a note with a higher pitch and can be performed from an open string to a fretted note or from a fretted note to another fretted note. You can also catch three notes by hammering-on another fret after the first hammer-on.
The symbol that denotes a hammer-on in tablature is an arch with a capital H over it, like this.
See below for an example of how the hammer-on is shown in tab. Using the A string as an example, the first pair of notes represents a hammer-on from the open A string to the 2nd fret of the A string. The second pair of notes illustrates a hammer-on from the second fret of the A string to the fourth fret of the A string. The third set of notes represents a hammer-on that spans three notes. Each hammer-on is sounded with a single stroke from the pick.