Review available curriculum options. Determine what you feel comfortable teaching, and how much of a mess any experiments might make in your home. Real Science 4 Kids is an effective science curriculum for parents who are inexperienced at teaching science. A Beka's curriculum is also easy to use, and everything you need can be found in one book per grade level. Apologia is helpful for parents who are strong in science but need some structure to guide the teaching process.
Research local co-op teaching groups. These are classes, generally taught by other homeschooling parents, who are more familiar with a given topic. Compare the cost and learning opportunities to determine if you should teach science yourself or enroll your children in a co-op class. If you choose to teach at home, develop a lesson plan that takes into account what you wish to accomplish with your child and teach according to that plan.
Take your child on trips to local science fairs, museums and other learning opportunities. These trips will complement your child's education give him a different perspective than he gets from his normal teaching.