Hello Phoebe,
So I presume you are staying with a friend or family in Woking? They might be the best people to advise you on parking there - but I can't imagine that there is anything risky about parking at Woking station!
Your train from Woking will then get into Waterloo station. It is a 1.2 mile walk from Waterloo to Guy's by the most direct route - you can check here. However they show the most direct route - a far nicer walk is to follow signs from Waterloo to the S Bank (about a 2 min walk) and then simply turn Right and walk along the path all the way along the S bank of the Thames. You will pass the National theatre, the Globe, Tate Modern and the Millenium Bridge, and come out in borough, where you will see Guy's signosted. This is a really interesting walk.
Your other option is to take a black cab from the station - but the price can add up.
There is a bus, the 381, between Waterloo and Guy's see here (Guy's and st Thomas's are linked hospitals - you are sure that your appointment is actually in Guy's?)
The nicest places to have lunch around waterloo are, IMO, on the S Bank. If you follow the signs for the S Bank from W'loo station, you will find a row of café type places alongside the Royal Festival hall, and then again underneath it, facing the river. Or if you are walking get lunch at the other end of your walk at Borough Market - again lots of nice cafes - just browse and take your pick. Neither are great shopping areas, though ilove the Royal Festival hall shop alongside the RFH, and there is a good bookshop underneath.
Trains out of waterloo at rush hour to a top commuter spot like Woking will presumably be jam packed 5pm-6.pm ish.
The S bank is a great place tohang out - there is usually something free going on in the foyer of the RFH, or on the terraces outside, you can sit and have a drink in the RFH on the terrace.
Or if you are walking back from Guy's to Waterloo, stop off in Tate Modern. It has great places to sit - balconies overlooking the Thames etc.
London is just a place, like any other, and despite what people say, most of us are very friendly and helpful to visitors, so if in doubt, ask - just don't be discouraged if you pick another visitor the first few times!
Good luck with your appointment.