I went on my first cruise last year and had a blast, I swore I would never do a cruise but after the experience including 2 days in a tropical storm I plan on doing one once a year. I did a 7 day Carnival cruise, my ship was the Splendor I heard some Carnival ships suck though.
Ive been on carnival twice and loved it each time. We had the Valor first and then the Magic recently. The Valor was a newer ship and was nice. We had an interior room and it was fine, but got a cabin with window on the Magic and it was nicer. The interior room isn't bad, you wont spend much time there other than sleeping anyway. You need cash for the ports, but most take cards now. No cash on the ship (for carnival anyway)
Relax and have fun. People suck no matter where you are.