Ask your parents politely and promise to improve on whatever field they deem you to be slacking. It could be in cleaning up after yourself, studies or even helping around the house even of you are already paying for it, they need to be assured that this phone will help you improve. You should not display childish behaviors such as throwing tantrums otherwise this will only ensure that they do not get you the phone.
If your paying for it, shouldn't you be able to have it? It is your money. If that factual info doesn't help, prove to them that you can be responsible with it. Do this by taking REALLY good care of it, like cleaning the screen, not cracking it, because they are easy to break. Don't get it wet, ect.
Don't ask for an iPhone right out of thin air, ask for a regular phone, like these images "http://bit.ly/1bfWRVg" Then when the contract is over, you can get an iPhone, this may take a long time, but an iPhone's only difference from the iPod Touch is the phone feature. This is mainly to show your parents that you are mature enough to handle a phone, and you don't need the phone for games, this worked for me, and I got it from a friend, now, in 6 months, I will get the iPhone 5s!