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.

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.

NOTE Make sure that the origin of the curve is in the same place as the object's origin.

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.

There are several ways to do what you want here's one:

Create an object 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. Then make sure you click on animate path.

Now your object will follow that path forever and ever and ever...

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.