As with most Slayer songs, this is in Eb tuning, so if you are in standard tuning then tune each string down by 1 fret to:

Low Eb : Ab : Db : Gb : Bb : Eb

Here we deal with the Intro, which is a long, atmospheric build up. Both Kerry King and Jeff Hanneman's guitar parts are shown here. The Intro can be divided into 3 main parts:

Part 1 - Starts with this easy riff using the Low E minor chord. Keep repeating after the double F5 strikes. Kerry King continues playing this riff while Jeff plays starts something a little different after a few times (shown here and here).

Part 2 - Next, Kerry King plays this clean guitar part. It's the same top 3 notes, but the bass note changes. It's a short repeating lick, repeated a lot. Halfway through, Jeff Hanneman joins in to play the chords (Part 3):

Part 3 - While King keeps playing the clean riff, Jeff Hanneman powers through this slow chord progression using full distortion. It starts off very easy, and then he starts using octave chords with slides. Nothing too hard, just make sure you keep that middle Db string muted throughout.