Hello Adrian, I don't think you will need an extra card, just an adaptor or dongle. Several makes available eg Netgear and Belkin. These plug in to a USB port on your computer - which I'm pretty sure your machine will have. The adaptor comes with a CD which contains the necessary software - you load this into the computer using the CD drive. It's all prtetty straight forward.
You will, of course, need to use a caravan site which offers wi-fi "connection" or use the free ones at some coffee outlets and the like. You may well have to pay quite a lot for those at the sites.
The other problem is that reception of the signal inside the caravan can be a bit of a problem as it is a big metal box, and even worse when the blinds are down as these are metalised. Once you have loaded the software you should see a small symbol of a laptop along the bottom line somewhere. If you have a good signal, the "screen" of this laptop will show green, if poor signal, yellow; if no signal, red. Get the computer up and running with this symbol and then move the computer about inside and outside the caravan until you get green or yellow.
You can sometimes help matters by buying a USB extension cable - female to male - put the adaptor in the female end and the male in the laptop. Then you can get the adaptor outside the caravan, e.g. on the roof (although it is not watherproof and should be protected e.g. with a plastic bottle with the end sawn off, or even well wrapped in klingfilm.
Purists will tell you that the maximum cable you should use is about 1.5m but I have used a 3m. cable with good results.
Finally, there will probably be other users on any site you go on who will help for the asking.