A graph is made using two different scales or axes, forming a right angle. The horizontal axis (x-axis) is used to represent the variable that changes in a consistent way, such as time, or in a way that you can control. The vertical axis (y-axis) is used to represent a variable that you measure but may not be able to control directly, such as a patient's temperature.
Each axis should be carefully labelled to indicate what it represents. To plot a graph, you put a mark at the point where the two variables in each measurement meet.
One way to remember which way round the axes go is to remember that X comes before Y in the alphabet and then picture entering someone's house; you go along the corridor (horizontal, x-axis) before you can go up the stairs (vertical, y-axis).