There are some general guidelines for making or using a wand. The wand should be a natural material, most commonly wood. Branches should not be torn from live trees; rather use branches found on the ground or from a dead tree. In a pinch, wood from your local hardware store will do. I read somewhere that metal is not recommended for wands but I don't recall the reason why. Again, any natural material will do.
If going the wooden route, you may wish to obtain a branch from a specific tree. I have willows in my backyard. Willows are especially magical. (Think Grandmother Willow from Disney's Pocahontas.)
The suggested length of a wand is the length of your elbow to the tip of your middle. For me this is about 18 1/2 in. You should use whatever is comfortable. Once you have the proper length, the wand can be decorated or left plain.
A wand is not necessary in witchcraft. It is an accessory which acts as an extension of your fingers and hand. Do not allow others to handle your tools and do not handle others' wands without their express permission. If purchasing a mass-market wand or one that you did not craft directly from nature, you may wish to cleanse it before using it. Use the cleansing method of your particular path.