Welcome to FIRST robotics! If by captain you mean the person who will be the leader of the team throughout the build period, then the best choice is the person who excels at forethought and organizing - and is willing to do what it takes to get the job done. You will soon learn that six weeks is not nearly long enough to build a robot - and having someone who considers the bot their "baby" is sweet because they will keep track of details. As a new team, you really don't have a pool of known personalities from which to choose. Don't worry though; many teams (mine included) do just fine without giving that title to anyone. Just make sure that the people willing to work the long hours communicate with one another. If you must decide, then I suggest that you have a teacher/mentor select the captain, as they probably have a good idea which students are responsible and pro-active (hey, maybe you

). As you become established, the natural leaders will surface and you will have a much easier time choosing.
If by captain you mean the alliance captain that will represent your team during the alliance selection and finals of a competition, then the role is vastly different. THAT person needs to know every robot at the event, and they must be able to identify which ones work best together. Sadly, alliance captains that win a popularity contest don't fare so well. They typically look at the seeding list and select the next available robot - with predictable and disastrous results. Alliance selection is dynamic, so you can't rely on a set plan or a seeding list. Our captain is chosen by the students and is usually a driver or scout who knows the strategies, robots that compliment ours, and robots to avoid (e.g. penalty-bots).
Good luck in the upcoming season(s) and welcome to the 1500+ teams of friends in FIRST robotics.