The green signs for CAT are more prominent; CAT (9 Euro) is slightly faster than S-Bahn (3.40 Euro, as of June 2009; will increase in July).
You can find the tracks for both services by following these signs:
On the way you will pass the "Billa" food store.
Remember what I said about CAT signs being more prominent? Don't worry,
you can still take the S-Bahn:
Now you have two choices: CAT to the left, S-Bahn straight ahead and then right. You can buy CAT tickets right here, S-Bahn tickets on the S-Bahn platform.
Gute Fahrt!