I purchased a Smokey Joe Silver as my first Weber to see if I could and how much I would enjoy using the Weber and charcoal. After reding the booklet I am confused by high, medium and low direct heat. Are the different temperatures determined by adjusting the vents?
Do I need a thermometer and if so which one do you recommend.

Bones

The temps are controlled by the vents and how much you let the coals burn down. You can judge them with the how long can you hold your hand over the grate test usually.

Check the section on Grill Temps in the link below.

http://www.bigdaddyskitchen.com/bbq&grilling/Grilling%20Guidelines.htm

By the way, the SJS is considered a portable grill, you can find plenty of info in the "Weber Portable Grills" section of the forum when it comes to specific questions about the SJS.