Have you gone to 'My Account' and reviewed the PDF invoice of your bill? It will have all the details and a break=down of costs.
Most first bills are higher due to activation fees, cost of the device up front, taxes on the device, etc.. The best way to find out why your bill amounts to more than double what you were expecting, is to check it in your account online or call Customer Care. If you are in your buyers remorse period, insist they remove any stupid fees or tell them you will cancel. Activation fees and other garbage like that need to go away.