Well, Thief13x, I am one of those intimidating s.o.b. s who conduct PPL Skill Tests...
First thing is - we WANT to pass you! We also know that you'll be tense and ill at ease on your trip - and may not be perfect. Don't worry - none of us were any more relaxed when we did ours!
I wouldn't really care whether you turn up in shirt and tie, t-shirt and jeans or a little summer dress (if you're a lady). Wear whatever you feel comfy in, because the trip will take about 2.5 hours.
As Pink_aviator said, you'll be expected to make the relevant Wx an navigation decisions, but no Examiner will make things deliberately awkward for you.
Start with a good Wx brief, then plan the nav route followed by weight and balance and take-off calcs. You might get asked the odd question during the walk-round checks, but I will normally let the lucky victim get a head start. Top tip - make sure you use the loo before you walk to the a/c! Then just a normal start, taxy and take-off (try to believe me!) before you set off on the first nav leg. The Examier should act like an 'intelligent passenger' and will just want to know the ETA at the first turning point, any update and if you're off-track, just describe what you're doing to sort it out. On the second leg you'll get a practice diversion - set off from somewhere obvious when you're ready for it. Once you get to the diversion, your responsibility for nav ceases until you're asked to take the Examiner back to the aerodrome a while later. My sequence goes something like - climb up to a suitable height for stalling, then level off and sort yourself out. A steep turn each way, then recovery from a spiral descent. After that a few stalls, then a PFL. Climb up from the PFL at best angle of climb with a few turns, level off. Then fix position by radio navaids, turn towards a nominated beacon and track towards it for 5 mins. After about 3 of the 5 minutes I'll give you the foggles to wear, ready for your 180 deg IF turn. Then back for a few circuits with an EFATO and a low go-around at some stage and it'll all be over before you know it!
Pink-aviator's description is exactly the sort of thing you can expect - she was obviously tense and nearly thrown off-balance at the start but then gave a settled and sound performance, from all accounts.
The very best of luck - let us know how it went when you've passed!