Some people experience nausea after taking birth control; thus, taking it at night will help. The best option is whenever you can remember to take it at the same time.
As soon as you start being sexually active, you should take birth control to prevent pregnancy. It needs to be taken at the same time each day. Some people prefer to take it first thing in the morning so that it's routine and they don't forget to take it.
It really doesn't matter when you take it so long as you take your birth control every day at around the same time. For example, if you eat breakfast everyday then take it with breakfast, or if you a bedtime ritual take it then. Just make sure you remember to take it every day.
Immediately. Though I would suggest at a time that you are going to remember that you have to take your pill. For Myself, I used to pick mornings since I would need to shower. For you, maybe before bed would be easier.
You shouldn't hesitate to ask your doctor any question you many have. Begin the first pill the first Sunday after you start your period. Take it while sexually active.