If you are just beginning I would recommend starting with a larger more terrestrial species and orienting the tank horizontally. The Phylobates genus and most Dendrobates tinctorius locales(such as azureus) would be a good start. Believe it or not the smaller, more “arboreal” genera such as oophaga and most ranitomeya require more space then you described and there are more caveats to their care than the previously mentioned genera. If you are determined to orient the tank vertically the only species I would personally recommend would be Epidobates anthonyi. They are small, relatively hardy and readily breed. I’d consider them to be more of an intermediate species but definitely doable as s beginner. Do as much research as you can and find a species you are comfortable starting with. You can always go up lol