Road trip from Nj to Florida?

My friend (18) and I (17) are planning to take a road trip late July into early August. We live in New Jersey and we are going to Florida. I know it is 18 hours to drive straight through, which is what we want to do. Is it hard to drive straight through with two drivers? With a small car, how many times will we need to stop for gas? Is it bad to drive overnight? Advice is helpful, Thanks :D