Moving type on a transforming path

All you have to do to make a text layer stick to a mask path is to add a mask path, expand the properties of the text layer and pick the mask path from the Path options.

The trick is creating a path that will loop. You'll need at least three keyframes. The first and last keyframes must be identical. Once you have the simple animated path hold down the Alt/Option key, click on the Mask Path stopwatch, and add this expression courtesy of Dan Ebberts:

if (numKeys >1 && time > key(numKeys).time){
  t1 = key(1).time;
  t2 = key(numKeys).time;
  span = t2 - t1;
  delta = time - t2;
  t = delta%span;
  valueAtTime(t1 + t)
}else
  value

If you need the text also to animate you can add any of the text animators. Most of them can be made to loop using a simple loopOut() expression. 

If you need any more help, please describe exactly what you want the text layer to do and give us screenshots if you can.