A circle with radius can be represented with the equation .
So, you can calculate the circumference of a circle by finding the arc length of the curve from (this will give you the top half of it), and then multiplying by 2 to get the whole perimeter.
In order to evaluate the integral, be sure you know your inverse trig rules.
Edit: For your second curve, are you given an interval? If not, is defined for all real , and the curve will be infinitely long (as we should expect, the improper integral diverges).