Welcome to the community!
Yes, you would receive your first bill shortly after you activated your account. Since you've received a bill, that would indicate you have post-paid services. You should note with post-paid plans, services are paid for the upcoming month. I understand that might seem counter-intuitive since post usually refers to after. I think the term might refer to receiving a monthly bill stating any accumulated additional charges during the month (see here). While the plan fee is charged prior to each month, overages and additional charges show after each month.
For each billing cycle, there would be additional charges from your previous month plus the monthly fee for your upcoming month (note that any financing would be additional). However, since it's your first bill, your additional charges would include prorated monthly fees for your two days of service (see here).
Hope this helps 😀