lindsay1982- Please see our cruising FAQ (last topic in the forum list) It will have some answers for you (and probably trigger some questions you didn't know you had)
Yes, you can use a debit card, (provided its a Master Card or Visa brand) but they will put a hold on the card. It will probably be as much as $500 for a single person. You are going on a 5 day cruise, the tips are $10 per person per day. So for your cruise they should only charge you $50 for tips (yes, they put this on your account day 1) I'm not sure where you get $120 from. You may be better off using cash as having a hold on your checking account that may not clear for a couple of weeks could be problematic. The CCL website says they will want $200 deposit per person for a 5 day cruise. Note that if you spend more than that while on board your sign and sail card will no longer allow you to purchase anything until you go to the purser's desk and add more money to your account.
Lastly, on a 5 day cruise there is 1 formal night. A suit coat is not required (although many men will be in them.) but he should at least have a dress shirt and dress pants. Look here for more info on dress code.
As Jim notes, it is $10 per person/per day for the auto tips, so if there are two of you going they will put $100 ($50 each) on your account pretty much the moment you check-in. There is also a 15% tip added to each bar purchase which is split amongst the bartenders and bar waiters.
The tips are pretty much the only money that the staff makes, and I think you'll find they more than earn them. I often give them a bit extra and also give some extra cash for favorite bartenders on the first day. It helps.