You may have inadvertently started in a paid version of the software. Make sure that you use this link, and again make sure you do not accept any offers for upgrades or promotions. 

Even if you are  in the free version you need to go though payment screens. You bill will be 0 and you will not be asked for credit card. It is a formality that software is required to have.  If you have not agreed to other services  you should have 0 invoice.