Can you give a little more info about what you're having problems with?
The simple answer is amount of charcoal and vent adjustments.
About a half Weber chimney full of coals will give you about 350 degrees at the grates. Don't shut down the top vent completely when trying to lower temperature. I would go bottom vent before top vent, but more likely you will need a combination.
For maximum heat when I'm doing steaks, I'll offset the lid a few inches for additional airflow. It really gets the kettle cranking. This also works well in more typical situations where you need more heat. Just adjust the offset as necessary.
Unless I'm doing low and slow, I seldom adjust the vents. I kind of just know how much charcoal with the vents wide open is going to give me approximately the temperature I'm looking for.
A cheap oven thermometer placed on the cooking grate work well to know what the actual temperature is where your food is cooking. The lid thermometer gives you the dome temperature, so there will be a variance at the grate. In the case of my Performer, it ends up being about 50 degrees.