How to buy a cheap iPhone 6?

So I've been wanting an iPhone for the longest time. The only problem is that I don't know how it works. My mom said she'll buy it(I'm 17), but she doesn't think she can because she needs good credit or something like that. Online it says $0 for the iPhone with the 2 year lease, but how does it work? And will it be expensive? I currently have an android and I'm with Metro PCS. Can somebody explain?

In order to get the zero down on the phone they do run a credit check. If your Mom doesn't have good credit, she won't qualify for the zero down plan.