I have only done a dozen weddings, its not my main business though I do love doing them.
Explain to the client you have a lack of experience. Being your first wedding, I would recommend that you do it as a freebie, that way, if you balls it up, they cant complain. That will also settle your nerves somewhat as there will be no expectations on behalf of the client.
Charge and balls it up and you could be in court coughing up thousands of pounds in damages, not only putting you off wedding photography but also damaging any future business you may acquire. If its free, they cant really complain. Just say to them, you do it for free and you want prints, that you would be happy to manage that for them (so you can earn a few quid out of it). Not sure where you will stand by charging, you will need to pay tax, possibly invest in an accountant - do it for free and its something you havent got to worry about - plus if you do get some good images, you'll have something to add to a portfolio plus know if wedding photography is something you're cut out to do.
I take 5 32GB memory cards plus another 5 32gb memory cards as backup. I normally fill up 2 or 3 of them. Maybe shoot 2,000 + images and then delete 80%+ of them, leaving you with a few hundred of the "best" shots. Anything you're not sure about, delete.
Its hard work, almost non-stop. You have to have eyes in the back of your head, keeping peepers open for any opportunities. Take water and something to eat. Have a break when they all sit down to eat. My back is usually in half by the time I've finished in the evenings.
Leave the processing for a least a day after you have taken the images, so you are refreshed (get some sleep!) and have a fresh-head on you when you go to process them. Dont physically delete everything, create a "not good" folder and shovel the stuff you're not sure about into that folder. Then when you've finished processing and you're 100% happy with your images, delete the "not good" folder, otherwise you may end up wanting to fish out some images from there that might be essential to capturing the event.
Dip your toes by all means - good luck and hopefully you will get some good shots
