best thing to do is spend some time going through the website for TNA - all you need to know is there.
The National Archives
as for travelling, there is a car park, but the Tube (District Line to Kew Gardens) is an easy option, with a 5 minute walk from the station (well signposted) and no parking/driving issues. If you can, go on the late-opening day - you may be glad of the extra couple of hours if the research is going well.