If you do nothing, you will be assigned a table and a serving team. For most people, that's enough. They don't have any preferences or need special services. However, if you have a server you like (or some look for recommendations because of past experience or things like "good with allergies" "good with small kids") you can call
DCL in advance of your cruise and put in a request by giving at least a first name and country they are from; last name is very helpful if you have it. Typically you should request only the server or head server or assistant server - not a whole team as they shift as crew contracts start and end. It is just a request and if they can fill it, your table will be in the requested crew member's assigned area. Alternately, once you are onboard, you can go to where they have dining changes and check the size of the table you were assigned and your dining team and make requests there -- although if you are requesting a server at that time, it is more difficult to move things around as their area may already be full.