Go to these two sites which have loads of faqs
http://www.cruise-information-center.com/defau lt.htm
http://www.cruisecritic.com/
As jno says , there are all sorts of different 'class' of cruises, but even if you go on one of the 'flasher' ones (say Princess or Royal Caribbean), there will only be 1 formal evening a week, 2 'smart casual', and the rest are casual (but no shorts!). The main thing do remember is that you DO NOT have to do anything you don't want to. There will always be an alternative dining area from the main dining room. Indeed, many people use the more cafeteria style dining areas to save dressing up, but loads of people (mainly women!) enjoy putting on the glad rags. Do not be worried, just pick somewhere to go and enjoy it! My first cruise was from San Diego, California to Miami via the Panama Canal. 5000 miles in 2 weeks...and didn't have to repack once!!