In a science fair project, there are three different kinds of variables. These are the independent, dependent, and controlled variables. The independent variable is the thing that you change- in this case, the type of food that's melting. Ice cream and frozen yogurt aren't actually the variables; they're materials.
The dependent variable is what you measure- in this case, the time it takes to melt. The dependent variable depends on the independent variable!
The controlled variables are things that you don't change at all- there are lots of these! For example, you wouldn't want to change the starting temperature- that might change the time it takes to melt. And you wouldn't want to change anything else, like the amount of ice cream/frozen yogurt that you're melting. That could change your results too.
I would also suggest looking here: http://www.sciencebuddies.org/science-fair-projects/project_variables.shtml
Good luck with your project!