Have a good look at the National Archives website before you go. They have lots of info on there regarding making a visit and it will help you plan your time.
Make sure you have some ID with you as the first thing that will happen when you arrive is that you will be issued with a readers ticket.
THe help desks at Kew are very good. Make these your first port of call after you register and they will immediately point you in the right direction.
Never be afraid to ask, it will save you time,usually there are a lot of knowwledgeable staff around to help you in each section, and they are happy to do so.
It is an amazing place, have fun.