Step 1: Getting associated with the circle.
r stands for radius, and radius stands for Really important.
Remember, radius really really is relevant.
Have you ever heard of diameter? Who needs diameter? Diameter is for fools.
Well you could use diameter, but then I won't be able to give you the neato way to remember the circumference equation in the next step (by the way, diameter is just radius multiplied by 2.)
To find the radius, measure from one point on the circle to the opposite side from that point using a ruler, that length is your diameter, divide that by two and BAM you've got yourself a pretty little radius.