The Grand is a nice choice for your first cruise.
As with all ships, the closer to the center of gravity you are, the less movement you will feel. Given that, an inside cabin as low in the ship as you can get, and as close to midship as you can get will give you the least movement. But I don't think you need to take it to that extreme, because the difference between the least movement and the most movement isn't that much in the grand scheme of things. Most of the noise and vibration is in the aft, and the worst movement is at the bow.
For your first cruise I'd skip the inside, you may not like it. Take a midship outside instead. Most of the outsides on the Grand have balconies... if you don't want that, you can get a midship outside down low on the Plaza deck...sort of near the purser's lobby. I think another really good choice for you would be a midship outside on the Emerald deck, in the E400's section; most have obstructed views, but if you are considering an inside cabin, then you probably won't mind. They're really good because they're cheaper, and you still get sunlight even though there's not as much of a view. Plus they're set back a bit (less movement). If you decide on a balcony, take midship on Caribe.
February is a great time for the Western Caribbean... it's never too cold for snorkeling down there.
If it's this coming February, you'd better hurry...they sell out fast and you may not have much to choose from.
