Hi Linda,
To get you going, I'll try and answer your questions.
For your fresh water you will need a suitable container such as an Aquaroll (which holds 40 litres). You connect one end of the pump to the inlet on the side of the 'van and drop the other end in the aquaroll; when you turn on a tap it switches on the pump and water flows!
The Gas normally lives in the front locker. The bottle (or bottles) are secured to the bulkhead and there should be some flexible piping that connects to the regulator on the gas bottle.
For waste water you will need a suitable container, such as a Wastemaster (which holds 38 litres) and probably some plastic piping and a Y-piece (available from any caravan accessories shop). You will find the waste water outlets (generally one for the kitchen and one for the bathroom) underneath the 'van on the side or rear edge - they tend to have little hinged lids on them. Anyway you stick a bit of pipe on each outlet, a Y-piece on the end of those and a single length of pipe from there to the Wastemaster! Just remember - when the Aquaroll needs filling, the Wastemaster needs emptying!!
Electrics. Generally, you will need a leisure battery. You will probably want a mains hook-up lead as well (25 metres). When you are hooked-up the mains will run your battery charger, any 240v lights you may have, your heater, your water heater, your fridge and your plug sockets. I believe (I'm sure someone will put me right if I'm wrong) your 12v lights and water pump will still run off your leisure battery, which is being charged by the charger - although they are probably running straight off the charger.
As to what appliances you can run when hooked up, it depends on the amount of amps you are getting off the hook-up. Hopefully you will get 16 amps, but it depends on the site and you can get a lot less. So, really you should be able to run most things that you are likely to want to take away with you - you probably won't be overrun with plug sockets anyway!
I hope this helps but don't hesitate to ask anything else.