If you're leaning from the very beginning and don't feel comfortable starting from standing, start here.
If you're comfortable with standing, you can go from either position. The first is basic, and the second is the CDTA Acro way. It's mostly stylistic, but it keeps your hands wider in a cartwheel.
Put your weight in your hands, and jump off your first foot, and jump to the other side.
You want your legs as split as possible so it is a cartwheel, not a roundoff. It also looks nicer with straight legs and pointed feet. Doing that polishes a cartwheel and makes your legs look longer!
The goal of a cartwheel is to be straight while upside down. You want your feet above your body, so if someone was standing in the way of your cartwheel, you'd be a straight vertical line like above.
When landing, your first foot to go up, is the first to come down. From the landing position, all you have to do is stand up and you're done!
This is the first step demonstrated when you're first getting the coordination of the cartwheel.
With the legs a bit higher.
And the full cartwheel :)
So here's two variations for when you have your cartwheel! This is a one handed cartwheel, and you don't put down your second hand.
This is a one footed cartwheel, and you don't use your first foot. You jump off of your second foot, circle it around, then land on that foot. Your first foot should never touch the ground.
Good luck! I'd suggest practicing on grass or carpet!