If you open your account today, then your billing cycle will be either today or tomorrow, you may, for one day have only a pro rata allowance, ie flext 35 - £180 allowance, divided down to an amount for each day, eg 180/30days = £6 allowance and £1.16 pro rata charge for the price plan.
Your billing date is the date your bill is printed and allowance reset. Your payment date will always be 14 days after that date, so if your billing cycle was 1st of each month your payment would be due 15th (on or abouts taking into consideration bank hols weekends)
Another hint, when you get your phone it takes 24 hrs for your allowance info to show on the 150 service, however you still have full access to this allowance but wont be albe to check.
TMobile are currently testing bill/allowance info via sms (of which im in the pilot scheme, its great!) so theres better than sitting through 150!
Hope this helps!