Dependin on the cruise line, usually your cruise card is used in place of money. Excursions go on your shipboard acct. and you pay off the shipboard acct. before debarking. All the cruise lines handle things a bit differently, though.
We just got off the CP and you never had to have real money to buy anything while on board the ship. If you don't pay at the beginning by credit card as mentioned above you'll have to pay the balance of your account either by credit card or cash (use a credit card at the beginning for your account it's so easy). Everything you buy off the ship (on land) can either be paid by credit card or cash.
PS - If you gamble they will take the "green" stuff and give you chips.
You will need money to pay for anything bought ashore including any private tours, cabs, etc you take. We carry our CC for any large purchases ashore but also carry maybe $100 in small change when we go ashore to use for things like taxi's, trinkets, food, or other things that strike our fancy.
You also need some small bills to tip room service if you use it. You also tip the porters at the pier in cash. Usually about $1. per bag I believe. If you are driving and parking at the pier you also pay cash for parking. We were on our first cruise in September on RCI's Grandeur of the Seas and we were able to get cash from the casino charged on your onboard account. I hope this helps. Feel free to ask any questions you may have on this board. I got tons of great tips and advice before we left from the wonderful people on here.
Have a great first cruise. We have already booked our second.