The height of the foot-bar is adjustable. There will be two to four notches on the inside of the frame that will allow you to adjust the height of the foot-bar. Your instructor will help you figure out which height is best for you.
The springs provide resistance from the machine. The rest of the resistance is provided by your own body weight. The springs hook on the spring bar, and different springs can be unhooked to increase or decrease the resistance. There is also a gear system that allows one to change the distance of the spring bar from the end of the carriage. This is sometimes used to adjust for a persons height. Again, your teacher will suggest gear changes if need be.
The springs are color-coded by the amount of resistance they provide. Usually, green is the lightest spring, then blue, yellow, and finally, the most resistance from red. Many levels of resistance are achieved through varying combinations of the springs. There are some variations in color-coding, but you will get to know the machines at your studio. Your instructor will tell you which springs to use for each exercise. Beginning students are never expected to know which springs to use.