There is no medication birth control methods made for cats. The only way to prevent pregnancy and the heat cycle is by getting your cat spayed.
There are hormone injections that can suppress the heat cycle, but the side effects of these are very risky and can even cause death. So most vets will not even do it.
Here is a link that will show where in your home state that you can get your female spayed for free to low cost. : http://www.lovethatcat.com/spayneuter.html