If you are seeking additional wave in your hair, there are multiple methods that will work.
For a longer term change, your best bet can be a perm.
For shorter term results, there are also other options.
With the right technique, you can use a curling iron, curlers, heating iron, or a straightening iron (wrapping your hair around the straightening iron as you would around a curling iron).
But, a method that is unfamiliar to many is via braids. If you have previously braided your hair, you likely have noticed that when you remove the braids your hair retains some wave, and can look really nice. That wave usually disappears fairly quickly.
To help your hair hold that wave even longer, before you put your hair in braids, add some product, such as Paul Mitchell Ultimate Wave, Paul Mitchell Hot Off The Press, Paul Mitchell Thicken Up, or Paul Mitchell Hair Sculpting Lotion. Then, once your hair is in your braids, press it with a straight iron or smoothing iron. Leave you hair in the braids for at least a couple of hours, and when you remove the braids your hair will show a beautiful wave that will last for awhile (exact timing is difficult to estimate as it depends on each person’s hair, but it should last noticeably longer than putting your hair in braids alone).