Welcome to Cruise-Chat
A three night cruise will be a wonderful start for you. Cabin D114 is in the front of the ship. Not knowing your sensitivity to motion you may feel a little motion, then again not. You have cabin decks above and below you so you don't have any noise concerns from lounges, etc. It should be a pretty good location.
Since this is a 3night cruise I would think you would only have a carry on luggage. But if you do have a bigger luggage you will pack it the night before and set it out in the hall before you go to bed. The cruise lines will give you instructions in a daily activity sheet called "pitter patter". The ship should arrive at 7AM but you can get up and go have breakfast. The cruise line will also let you know what time and where you should go for disembarkation. Relax, the daily Pitter Patter will give you a lot of information. The ship has a Guest Relations desk you can always go and ask questions if you can't figure something out.
a three night cruise is an excellent way to see how you like cruising; and Sapphire Princess is a wonderful ship--one of my favorites.
D114 is a good cabin in that you have passenger cabins above and below you. Being all the way forward you will have a pretty good walk to just about everything except the Promenade Deck which is two decks below you. But all that walking will just give you a healthy appetite so you can eat that much more.
The way disembarkation works is this: you pack your suitcases and put them in the hallway the night before--everything but what you are going to hand-carry. Then you collect your luggage after you leave the ship. OR, if you want to carry ALL your own luggage, you can be own of the first ones off. Unless you have an early flight out of LAX I would avoid the self-disembarkation--it is usually very chaotic. It's much nicer to go have a nice sit-down breakfast in the dining room around 7:00; then collect your things from your cabin and head to the lounge to which you have been assigned. Everyone is assigned to one of the public lounges and a time to be there. Then at the appropriate time you will be escorted off the ship.
The most important thing to pay attention to is the Princess Patter; it's a newsletter placed in your cabin each night that tells you in detail what to expect the next day. Read it cover to cover with a marking pen in hand.
Any more questions, please just ask. I live in L.A. and have been on Sapphire Princess several times and would love to help make you a cruise convert.