wrote...
I just realize my toilet is leaking at the connection of supply line to
the toilet. I've replaced the supply line but it's still leaking. I
don't think is the filling valve but I'm no plumber and a first time
home owner. When I turn off the water, the leaking stop, so the water
is not coing from the filled toilet, could it be the nut where the
supply line connecting to the toilet?

There is a seal on the nut as well as a seal for the assembly to the toilet
tank. Get a replacement fill valve assembly, which should include both seals.
You may want to replace the flapper valve while you're at it. Once one seal
starts leaking, the others are likely to follow soon.