Do you have the plan activated already? If not, you probably want to do this before you buy the iPhone 5S from Apple (it can be the day of), as this plan can only be activated online or at Wal-Mart. You would have to renew the plan every month, but I think that you can set up an auto-pay to help you out with this.
Great questions! It's good to know what to expect.
First, the plan. That $30 plan you're referring to is only available through Walmart. You'll need to purchase the activation kit from them directly.
Once you have the activation kit, we'll activate the SIM card in the kit and then when you get your iPhone from Apple, we'll just switch the SIM card number in our system to the nano-SIM that's built into the device. Then, you'll be all set.
You can set up auto-pay or auto-refills through T-Mobile.com once the account is fully activated.