There is no way to know in advance how smoothly an Embarkation might go. I always try to get there about 11:15 AM to 11:45 AM. Sometimes you beat the crowds, sometimes you are part of the crowd. Everybody eventually gets on.
You can't get into your cabin until 1:00 PM. They need time to turn over the cabins for cleaning and preparation. Take what ever clothes you wish to wear including dinner close, in your carry-on because you might not get your luggage until 7:00 PM. It usually arrives by 4:30 - 5:30 PM but sometimes it lands on the wrong floor and you keep waiting for it.
The first dinner is always casual so almost anything is OK but it must be within your comfort zone.