I think a good way to start your first math lesson is to mention an example which all students can relate to, like rollercoasters or movie theatres or Facebook. Then find a way to show the math involved or have the students figure out something.
from the site: "Last week, we asked, "What's your favorite first day of school activity?" Here are eight great ideas to break the ice, get students to warm up to one another, and establish rapport on the first day of school. Carol VanHook uses Stand Up, Sit Down!.....: