My new syphonic toilet is allowing water into the tank constantly

Is this a standard gravity operated unit or one of those new pressurized tank toilets?

If a standard toilet is running constantly without overflowing the flapper isn't seating properly. That could be the result of the chain being to short (holding the flapper off the seal), the flapper being misaligned, a dirty flapper valve seat or the rubber of the flapper being old or poorly manufactured.

If the toilet is overflowing as the water runs the float may require readjusting. Standard floats have a rod length adjustment as well as a rod that can be bent. I prefer the Fluidmaster 400A fill valves with the float that slides up and down the fill tube though. Fluidmaster | The Most Popular Name in Toilet Care then use the menus to navigate to toilet fill products.