The general rule of thumb, the higher up you go the more expensive it is, not necessarily any better, may be closer to some of the areas of the ship you spend time at?? For less feel of the ship moving, midship is always good. Just choose a place on ship you'd like to be, I personally like to be somewhere near stairs/elevator. Some cruisers like to be closer to the dinning areas or the casino. Are you looking at an inside/outside or balcony cabin?
Here is a link to some great tips for a first cruisers, lots of good info to read: