The equation is x^2 + y^2 = 9
There are different ways to do it.
Cartesian method (y = f(x))
You will need two functions because of the limitations of the cartesian coordinate system, obtained by rearranging the circle equation in terms of x:
$\displaystyle y = \sqrt{9 - x^2}$ and $\displaystyle y = - \sqrt{9 - x^2}$
Polar method
This one is simple.. $\displaystyle r = 3$, since the radius of a circle is constant.
Parametric method
Use the trigonometric functions $\displaystyle X_t = 3 \cos{(t)}$ and $\displaystyle Y_t = 3 \sin{(t)}$.
You can change the type of graphing method in the setup menu of your TI.
Umm.. whichever, they all produce the same circle. It's up to personal preference and context.
I would personally recommend the parametric method because it's easy to change the center of the circle if you need to (as well as the radius of course).