To find the circumference or area of a circle, you just need to know the radius, r. That's the distance from the center of the circle to the edge. Or, if you know the diameter, the longest distance across the circle, you can divide that by 2 to get the radius. You'll also use the number π (pi), which is 3.1415...
Circumference: When you know the radius, insert that number r into the formula for circumference, c=2×π×r.
Area: Insert the radius r into the formula for area, a=π×r^2 (that's pi times radius-squared).
Here is a good resource on circles: https://www.khanacademy.org/math/basic-geo/basic-geo-area-and-perimeter/area-circumference-circle/a/radius-diameter-circumference
Hope this helps!