It isn't advised to have your teeth whitened until you older, as the effects of it only last around 6 months, so you will probably want them done again and whitening your teeth isn't very good for them. Some dental practises may put their own age rules on when you can have your teeth whitened but there isn't a legal age.
After you have your braces off your ortho will clean up your teeth and I'm sure they will come up nicely when he does this, he will spend quite some time getting any scale and plaque off your teeth that has been unable to be cleaned due to your braces. I think you will be surprised!
Whether whitening hurts or not is completely down to you. You will tell your dentist how white you want your teeth and that will determine how much bleach you put in your tray. Whitening your teeth is like having a mouth guard, your dentist will take impressions (like when you had your braces) and they will make you a grinding guard, then at night you will put the bleach in the tray (grinding guard) and sleep with it in.
Teeth whitening should not hurt, however if you put too much bleach in the tray your teeth will become very sensitive, as in; If it is a windy day and you smile whilst the wind is blowing you will feel it and it will hurt a lot! This is a sign that you are using too much bleach and you need to calm it down.
Your dentist will show you how to do it all yourself and then it is reliant on you to complete it at home.
But... After you have your braces off your ortho will (or definitely should) give you a retainer to sleep in and you will have to wear that every night for a couple of months, then every other night, then every week, every month so on and so forth until your teeth have locked into place, as if you have your train tracks removed your teeth can and will still move. So you'd have to wear your bleaching trays during the day and your brace at night (or vice-versa), which would not be ideal as you may as well still keep the brace, you're not getting any freedom.
As I say, teeth whitening costs a lot of money and only lasts for around 6 months - a year if you're lucky - so if I was you I'd wait for a while and use 'Oral B Pro-Expert' toothpaste until you only have to wear your retainer once every month or so. Enjoy having your braces off for a while before you put something else back in there!
Hope I helped.
P.S. Sorry for the essay!