The method for adjusting a toilet float will vary depending on the type of fill valve you have. Older toilets may still have a ball that is attached to a rod. This type will have a screw near the valve that is used to adjust the height of the float. The water level in the tank should be adjusted so that it is near the top of the fill tube in the center of the toilet.
Turn the screw (likely counterclockwise) to lower the float. If the screw is stuck or you are afraid you'll break it, you can use two hands to bend the rod for the ball cock. Flush the toilet and see where the water stops when it refills. It should be just below the top of the fill tube for the flush valve.
Adjusting in the opposite direction will lower the water level in the tank. This is needed when the fill valve will not shut off, running constantly.
A fluidmaster type toilet fill valve has an integral float the has an adjustment rod on the side of the assembly. You should be able to pinch the spring clip and adjust the float up or down. Some of the newer ones have a screw type that can be adjusted with a screwdriver.
Move it down a half of an inch for a toilet that is running constantly. Let it go and see if the water stops running. Still running, adjust it down again. When the water stops running, flush the toilet and see if the tank refills to the proper level. Refilled OK and the water stopped running? Yes, good job, problem solved.
The 'Next' page covers 'Draining a Toilet Tank'. The 'Previous' page is the 'Introduction' to this series of articles on 'Repairing Toilet Fill Valves'.