How does the iPhone with tmobile plan work?

Like how do you do it and how much does it cost if it does

There is no tmobile plan for iphone

Its just the normal plan as far as I know. I dont think it has an extra cost. You have to go to tmobile and ask for a Iphone Simcard(If we are talking about the Iphone 4 or 5) they will give you one and switch your account over to it so you can use the Iphone.

TMobile has already announced they are getting the iphone in 2013.

You will need to chose any T-Mobile plan that you want and then pay all the fees including the one for the SIM card. Once you have done this, you simply place the SIM into the iPhone; the iPhone has to be unlocked and compatible with T-Mobile. You can buy factory unlocked phones (including the iPhone 5) from Apple. Once you have placed the SIM card into the phone, you may need the T-Mobile rep to configure settings for the device and all things should work afterwards. The iPhone is not fully supported on T-Mobile so depending on which area you're in, you may only get 2G speeds instead of 3G or 4G (HSPA+)