Here is a great video to assist you in starting your lessons on circles. We all know the standards have changed and this one specifically is a head ache due to the algebra 2 that is involved.
CCSS.Math.Content.HSG-GPE.A.1 Derive the equation of a circle of given center and radius using the Pythagorean Theorem; complete the square to find the center and radius of a circle given by an equation.
Most of us have students that have not learned how to complete the square or even mastered basic algebraic proofs. Deriving the equation of a circle is impossible without being able to complete the square. Below is a great video on how to teach them this algebra lesson so you can teach this geometry standard.