Green wood is not suitable and sound conservation practices mean never cutting a living tree. Hardwoods such as ash, oak, and maple are good choices if you can get them. Old Robin Hood would have preferred yew or sweet chestnut. Bamboo is light and strong and, in some areas, diamond willow and saskatoon are popular. Poplar, aspen and birch are okay, although I find them a bit heavy.
Conifer saplings are usually straight, light, and strong.
Use whatever you can find in your area. When you are ready to strip off the dried bark, a draw stroke works best. The length of your walking stick is pretty much a matter of taste. Some like a short, light stick just above waist level. Others choose one about chin height.