Some people's hair will naturally grow into an afro if left on its own. However, if you don't have very frizzy or curly hair, you will need to do some work on it to get it into the proper shape for an afro.
Grow your hair until it is at least 4 to 6 inches in length.
Wet your hair and comb it upwards to give it body. You can also put a little bit of jojoba oil into your hair to give it added body.
Braid your hair into breads that are no more than half an inch thick. Braid them as tightly as possible.
Sleep in your braids for one, or even two, nights to give them a frizzy curl.
Take out the braids and tousle your hair upwards, again with a little jojoba oil, to give them added frizziness and volume.
Curl your hair with a curling iron if you really need to give it added kinkiness or if you are having trouble getting it to stand up on its own.
Use an afro pick or a rat-toothed comb to tease and fluff your hair up and outwards. You can give it extra fluffiness and volume by using a hot blow dryer at the same time.
Carry the afro pick around with you and use it to fluff your afro up when necessary.
Avoid showering or bathing your hairstyle for as long as possible, as this will return your hair to closer to its natural state, requiring you to repeat the steps again.