Toto Toilet Leak

Water won't stop running. I've messed with the little plastic screw but it keeps slowly filling. Toilet is about two years old. Any ideas? Thanks.

If the water is overflowing into the overflow tube than the fill valve needs to be replaced.
If the fill valve is running and the water is not over flowing into the overflow tube than your losing water through the flapper and that will need to be replaced.

You need to find out the model number of your toilet to get the right replacement part.
If you don't know the model than if it is a 1 oiece toilet it will be stamped in black ink on the inside back of the tank and will say someting like "CST854" if it is a 2 piece toilet it will be under the tank lid and should say something like "TCU706".