We all started at the beginning, knowing too little and with MUCH to learn.
Very few "pet shops" carry Koi with any potential for anything lasting, so take Dick's advice and learn all you can from these little guys. They will not thrive in an aquarium, as Koi need a much, MUCH larger environment to thrive, but you can still learn on the cheap while you construct your pond.
There is a pond construction forum here, so go look around and soak up all you can. You'll learn more from others experience than you can get answers for one post at a time. Lots of good info to be gleaned just by reading, and then you'll have a better idea of how to start out right when you start digging.
Whatever you do, don't put a shovel in the ground before you read...
Don't hire a contractor before you read...
Don't start buying equipment before you read...
You will thank me later

(after you read that is)