I did a pond that was quite easy to do... for people not to skilled at brick laying....
The easiest way to do it is as the above mentioned poster has shown is with cement block. Now if you are not a trade mason and good at laying brick as I am not... then your neighborhood big box store sells a product in the caulk section, that is meant for gluing masonry capping stones onto the tops of walls. This product, don't have the name off hand, but from the same company that puts out liquid nail... works extremely well.
So, with my koi pond, rather than setting mortar, I actually glued the blocks together... make sure you stagger the blocks, to give added support. Then after gluing and creating the whole structure, because the blocks were staggered, I then went back and on every other hole I hammered down through the hole into the ground a piece of rebar. Then in-filled with cement... this made the structure very strong and tied it all together. 1200 gallons of water has a lot of weight that will want to push out... you don't want it to fail.