How do I convince and know if I'm getting an iphone?

Ok so I'm a teen girl and I really want and iphone 5. My dad has t-mobile as the carrier and its only $50 to add another line. I also insisted on paying for almost half of the cost (half of $150) so what do I have to say? I already mention it and when I'm on the computer and get up to leave i make sure I have the t-mobile website showing. So sometimes I feel like I may get it for my birthday (which is in a month!) because of the way he respond to me… So umm I kinda need advice

Ask for an older model maybe? Like the 4s so it will be a little cheaper. And maybe even ask him if he has any work to do around the house and such. Or suck up to him ha lol. Tell him your his little girl and be way too nice.