If you upgrade to a paid subscription during your free trial you’ll only get invoiced once the trial has run its course.
If you upgrade to a paid subscription after your trial has expired, you will be invoiced immediately.
In either case, your data will remain intact as you move from trial to a subscription, even if you are upgrading from Starter edition to Professional.
Note that subscriptions are based on a 12 month contract (with the first month free) and will auto-renew every month on the same day that the trial was started.