After your initial deposit you can make payments up to 90 days before the cruise is to take place. You can also prepay your shore excursions and your gratuities too. This might make things easier for you as there is no interest charged by the cruise lines to do this.
Since you are not flying in.....you can take all the luggage you want but my advise is keep it simple as most ships do have laundry facilities aboard.
Since this is your first cruise.....I would book a port intensive cruise so you can see as many places as you can.
Last but not least, enjoy planning you vacation! Do check as some cruise lines will not let anyone book a room that does not have at least one person who is at least 25 years of age. (I believe I am right in saying this)