Getting Billed but never activated my phone.

Why did you start a new account with ATT if you weren't ready to switch?  

If you want to switch service you port your number over once you current service is no longer wanted.  

Did you just order a SIM card or did you select a wireless plan? Billing starts as soon as you select a plan. When you activate, as you have found, has nothing to do with it and there is no way you will be able to use your current phone number with that SIM card. If you want to keep your number, you will port it and add a new line to your account.

I thought i ordered just a sim card but apparently not. I never got notice that the line was activated and was being charged for service.

When you order even just a SIM card for a bring your own device, it will have you pick a plan.

Once the item ships, the line is put in a reserved status and ready for activation. Once you receive it, you need to activate it. However, if you don't activate it, it may eventually be activated by the billing system.

You should be able to check the bill to see the first day the line was activated. Since you received it today, I'm going to esstimate that it was activated about 7-10 days ago, but I could be wrong.

You may be able to call customer service and see if the activation was done in error or if it was an intentional activation by the billing system. They may be willing to work with you with an adjustment for the bill, but I wouldn't go into the call expecting it as by signing up for service, you agreed to the Wireless Customer Agreement that covers these sort of things.