There's lots of information on this site. Check out the Learning Center and Learn the Dances.
And since I'm a teacher, I'll give you a little extra advice: The best way to start researching a topic is to ask yourself some questions about it. What do you want to know about ballroom and social dancing? What do you think the people who will see your poster would be interested in knowing about it? When you have a list of questions about dancing you would like to know the answers to, then go looking for the answers to them. That will result in a much better project than a bunch of random facts.
Lots of colleges and universities have classes and clubs for ballroom dancing. If you google "ballroom dance" and "university." you'll find lots of solid information.
Good luck on your project!