Sophie, welcome to Cruisemates,and congrats on your first cruise...here are some tips for you to use a a guideline.
Don't book with the cruise line, use a cruise specialist agency. You will be sailing during [Sept] hurricane season, so the prices should reflect a slightly lower price, allowing you to maybe get a btter cabin, for a lower price. Your budget is tight, will you need air fare included? Also make sure you ask your ta about travel insurance.
The eastern itinerary, St Thomas, St Maarten, Nassau, is best known for shopping...the western itinerary, Cozumel, Jamaica, Grand Cayman, Roatan etc, is well know for great snorkeling, diving, and water sports. Do you have a dream port you always wanted to visit?
At this time of year, many ships are in Alaska, and Europe, so your selection of ships is not optimum....give us some of your thoughts, so we can give you more of ours....