There are several ways to do what you want here's one:
Create an object, then and a Curve->Circle to act as the path for your orbit.
Select the object and add a Follow Path constraint and select the curve as the path to follow. Make sure you click on Animate path.
Now your object will follow that path forever and ever and ever...
To control the speed of the orbit select the curve and change the Path Animation value to a smaller number of frames to go faster or a bigger number to go slower.
