As a new subscriber you are billed for your first box at checkout. Then, you will be billed on the 1st of the month after your first box ships.
So for example: You purchased a box on 23rd of April. So that means you are getting the May box. Another, is that you purchased on the 2nd of April. Then you are getting the April box.
Remember the cutoff for the current month's box is the 6th of that month. Anything purchased after that day, is for the next month's box.