Pick up a book like this one. There are also ones that are specific to the region (i.e., eastern v. western Caribbean. Try the public library - doesn't make any difference if it is the 2008 edition. The book will compare cruise lines and answer a lot of pretty basic questions.

Our experience with seat assignments in the main restaurant was that you have 2 basic choices..."assigned seating", in which case you have the same seatmates for the entire trip, or "open seating", where the host assigns you to a different seat for every meal. Then there is buffet-style, where you line up, eat entirely too much food, but to get to pick your own seat. I didn't have any opportunity to pay anybody off to get a better seat.