A good theory is a theory which examines the evidence that is collected from the real world and provides an explanation which explains why that evidence is there. it will then go on to make testable predictions about the theory (it "theorises"). If these predictions are shown to be accurate, then it is further support that the theory accuratly describes the real world.
For example, the theory of gravity explains why things fall the way they do, and it also makes predictions about the future. We are able to predict how quickly an object will fall, what force it will hiot the ground with, etc. A famous example of predictions in the theory of gravity was the experiment conducted on the moon when a hammer and a feather were dropped at the same time and they fell at the same rate.
The theory behind the Periodic Table of the elements was also able to make predictions about the nature of elements that had not been discovered at that point. These predictions were later shown to be completely accurate.
Creationism does make an attempt to explain the past, but it makes no predictions whatsoever about the future, so it cannot be called a theory.
The Big Bang theory does make an explanation about the past, and it also made predictions. The Big bang theory also predicted the expansion of the universe as well as the cosmic background radiation.