Turn the valve off down there on the wall, get you a small bucket or empty trash can, unscrew the supply line from the toilet and point it into the bucket, turn the valve back on slightly to see if the supply line is clear.
If the supply line is clear and flows freely you have a problem with the valve in the tank. It may work by a ball that floats up, pushing in the center or the top of the valve to shut the water off. Or it could be a float that slides up a post causing a wire to push the actuator on top of the valve.
You may have to go to the Home Depot, Lowes or walmart for a new valve that matches the old one. Take the old one off by unscrewing a rather flat nut off the bottom of the tank ( crescent wrench or channelock sized )and take it with you so they can help you with it.
The instructions come with the new one and shouldn't be any problem for you but, if you have trouble just post back and I will help you.