Make sure the dampers are all wide open when you're starting the fire and getting the charcoal burning, regardless if whether you're building a direct or indirect fire. The charcoal needs the oxygen to burn. You don't need to open them further when you add charcoal to a good fire.
Once everything is covered with ash, partially close the bottom vents to adjust the heat -- but never close them all the way or your fire will go out when you put the cover on the Weber. Leave the vents wide open if you want a really hot fire, say for getting a nice char on your steaks. Leave them about 1/4 open if you're cooking something like chicken using the indirect method.
I disagree with Bob on one small detail -- I do (or did when I had a Weber) close the top vent partially if I want a really slow fire. It's all about air flow -- the more air, the hotter the fire. Watch your food, learn to judge the temperature with your hand, and adjust the vents and add charcoal as needed. To judge with your hand, hold it 6 inches to a foot above the grill; if you can't hold it there for more than 2 seconds, the fire's very hot; if you can hold it there for 5 seconds, it's medium; if you can hold it there for 8 seconds, it's low, etc. YMMV.