it is my experience that seen from a distance, a ship for whatever reason looks bigger than when you're standing in front of it. As this is your first cruise, the ship might look huge nonetheless.
There will be plenty of check-in counters, sometimes sorted by letters (head to the counter with your last name's first letter then), and there are usually extra counters for suite guests, past passengers and sometimes non-US passengers.
Check-in is similar to what one experiences at an airport, and with today's security measure, boarding a ship is actually very similar to boarding an airplane.
One important hint: Make sure to complete all forms required (included in your travel docs) at home. Lots of people only do this once inside the terminal and it makes check-in and boarding more complicated than it should be for them.
On the other hand, you can relax and look forward to a great cruise.
Once onboard and in your cabin, check your escape route so that you know it in the unlikely case of an emergency (you will have to use it during the mandatory lifeboat drill) and if you would like to make any appointments (alternative dining, spa treatments etc.) make them right then if possible to get the maximum choices.
Once you're settled - relax and enjoy!