Former Captain of Robolions 3397 here, now an alumni. Congrats on being elected Captain, I will share what I can with you.
I will start off with what I feel is the most important part of being a Captain,
respect.
Your teammates have elected you Captain, so that probably means they already have some form of respect for you. Having the respect of the people you are leading is the most important part, in my experience, of being a leader. Keeping respect is not always easy, nor does it always make you the most popular person on the team. To start off, it is important you keep this picture in mind as you are a Captain.
(Sorry about bad formatting here, if someone can teach me how to create one of those spoiler boxes I will hide the image so it does not take up as much space.)
Be a Leader, not a boss. Just because you can tell people what to do does not mean you are exempt from the work.
Sometimes you have to make an unpopular decision, sometimes you have to break bad news, sometimes you have to give someone a job that they do not want to do. Sure, these things might make you a bit less popular at the time, but they need to be done. Don't shy away from making unpopular decisions, just because you want to be loved by your team.
This sounds harsh, but is not quite as bad as it sounds. This is a high school robotics team, and not an army or such, so hopefully nothing too crazy or controversial should come up
.
Also, as a team captain, you have a much bigger time commitment to the program than the rest of the team. Inside robotics, you still have to keep track of all the building, programming, chairman's work, etc, but at home you often need to spend late nights working with various team members on various projects. (Examples of this would be when the Chairman's people want your advice on the essay, or the build team needs you to stay up to review the CAD they are working on.)
As others have covered, communication is important, so I will not go too much into detail on that. All I have to say is that you are often the median for communicating between the different sub teams, or the youth/adults.
Don't be afraid to ask for help! I assume this is not an issue though, as you did post this thread. Just remember that mentors will often be able to help give you guidance on decisions.