This is a free online video lesson for beginning guitarists by Peter Vogl. This lesson will explain how a slide is represented in standard guitar tablature. You will learn which way the slide should move and what strings and frets to play them on.
Watch the lesson: Quicktime or Flash
Here is the example tablature we will use for this lesson:

Once your done with this lesson take a look at all of our lessons on reading tab.
The Beginner Guitar Download Package by Peter Vogl contains six video lesson for the absolute beginner guitarist. You will learn how to hold the guitar and pick, how to play notes, beginner chords and strums, and how to read tablature. These lessons are designed for someone who has never played guitar before and will teach them all of the necessary fundamentals for becoming a guitarist. The included lessons are Beginning Guitar Lessons 1, 2, 3, 4, 5 and 6.