Since leopard geckos are ground animals and rarely climb, the tank will need to be long and wide (not tall and narrow). You will need to make sure that your gecko cannot easily climb out of the tank, so a tank with glass sides would be a good choice since leopard geckos cannot climb glass. Most leopard gecko owners recommend rectangle glass fish tanks/aquariums since they are easy to get and meet all the requirements.
Do not use wire or mesh cages for your leopard geckos. Wire cages will not hold heat very well, are easy to escape from, and you risk your leopard gecko getting his/her foot or toes stuck in the wire.