Welcome to your first cruise!!!
You can have a great time without spending a lot of money, but your can't go without spending any more money. Bringing a wide mouth water bottle with you that you can fill with ice and water, lemonaid, or tea at the buffet will save a lot of money on sodas and booze... You can refill this any time during the day.
Do be sure to budget $10.00 per person per day for tip money for the waiters & cabin stewards. They will be working very hard for you and, like waiters in our restaurants, depend mostly on tips for their pay (like it or not, that's the way the cruise ship industry operates). For 2 of you on a 5 day cruise, this is $100. You will also need to tip porters getting on and off the ship, $1-2 per bag.
Other extras you can budget for: photos, $8-$20 each. Shore Excursions, casino, etc... There are 1000 things to spend your money on while on the ship. The trick is budget & self control.
Have a great time.