What are the best rules to set for my child?

Focus on safety. Set rules surrounding safety issues- like how to cross a street, not talking to strangers, etc. These are the most important for your child to learn well and early. Practice them often to help with reinforcement. Other important concepts include learning manners (please and thank you) and how to play nicely with other kids (no biting, sharing).
Discipline . Child discipline differs by age, and is too much for 400 letters. Try: http://www.healthychildren.org/English/family-life/family-dynamics/communication-discipline/Pages/Disciplining-Your-Child.aspx.