My DS was a little older - maybe 3.5 years old when we started. It started by having him wash and care for his own genitals at toilet and bathtime. Basically, we told him that that area was considered a "private part" and it was his job (and his job alone) to take care of it. Mommy and daddy would always help if he needed it, but he shouldn't be sharing those private parts with others.
DD has been getting the same discussion lately, and she is about 2.5
Jen - SAHM to James
and my sweet baby Thomas