That's a general question that's difficult to answer without knowing what type of product or installation you're working with, but I generally try to imagine that I'm the one that's going to be using the drawing to produce the part and that helps me decide. That's a good practice for more than just where to start the ordinate dimension too, by the way.
With that being said, it really does depend upon the situation. For long tubing with holes, I of course start at one end. For installations that are partly above grade and partly below I'll usually start the ordinate dimension at grade and go both ways.
The general rule of thumb is that the ordinate dimensions start at the datums.
The integral features which need control (datums are used here, as Dan wrote) should be the point of dimensional reference.
Might be good to invest in some GD&T training.
As was said Datums. If none, where will the measurements need to come from how is it built. In other word how will the guys on the shop floor tell if it is correct.