The Earth takes one year to complete a full orbit of the Sun, most people know that. However, the exact amount of time is said to be 365.25 days and that is why we have a leap year every 4 years to make up for the extra day. Read More about the year here.
Now we know how long the Earth takes to travel around the Sun, now we must move on to mathematics.
To find the circumference (or orbit) we need the diameter and then to multiply it by Pi.
According to wiki answers the Earth varies in distance from the Sun each cycle but the average distance is as follows :- 92,955,887.6 Miles or 149, 597, 870.7 Km