When do you first brush your child's teeth?

As soon as they come. It is a good idea to begin massaging your kids gums even before they have teeth, so they get use to the idea you will be in their mouth on a regular basis. If you wait until they have a bunch, they are likely to have begun to develop the "i don't wanna" attitude & you will have a constant battle. Simple cleaning with a wet cloth or small brush with floride free paste is a good first step.
Now. Before teeth come in wipe gums gently with moistened cloth. After teeth erupt begin brushing very gently with soft toothbrush. Once child can spit out, use very small amount of non-fluoridated toothpaste, eventually graduating to fluoridated toothpaste. See a pediatric dental specialist @ age 1 for examination (to make sure development is normal), guidance and advice. See orthodontist @ age 7.