Let's calculate that. First of all, we know that, in general, the distance you travel equals the speed you travel multiplied by the time (duration) of travel. If we reverse that, we get that the average speed equals the distance traveled over time.

We also know that the time it takes for the Earth to go once around the Sun is one year. So, to know the speed, we just have to figure out the distance traveled by the Earth when it goes around the Sun. To do that, we will assume that the orbit of the Earth is circular (which is not exactly right, it is more like an ellipse, but for our purpose, a circle is close enough). So the distance traveled in one year is just the circle's circumference.