I'm going to operate under the assumption that you've already looked into Box Jellies and understand the risks and not try to dissuade you. Though I do agree that starting with Moon Jellies might be a good option because they are probably cheaper, and it'll take some experimenting to figure out how to care for them.
For the tank, toddnbecka already mentioned that you can't have corners. There are two generally accepted standard designs. The first is a cylinder, with water flow diffused through the entire bottom (through gravel), filtered, and directed back up through a central return line, usually with bubbles and colored lights. This can actually be done by modifying several cheap tanks you can find at places like PetSmart.
The second is what is called a Kreisel (German for circle) tank, which is a very short cylinder on it's side. So you have a flat front and back viewing pane, and the top/bottom/sides are all one circle. Water flow is directed along the outer edge, which keeps the jellies suspended in the middle. Jellyfish Tanks & Accessories available from Jellyfish Art | Buy Now
has examples of both types, and information on their care.
Because they are not very efficient eaters, water quality can be very difficult to maintain. So, as with any saltwater tank, get the largest you can afford. I'm afraid I don't know how to tell if they are being under or over fed, or if they are healthy, beyond "they're still moving..."
Hope this helps!