Does it look like this?
If it stops leaking soon after you fill up the tank, you may have just overfilled the tank.
Not a big problem.
If it continues to leak after you have parked the bike, you have a carburetor leak.
If it's a large puddle (like a flood) of gas, it is likely the float valve stuck open.
If it's a smaller leak, as in the photo, it's the o-ring on the float valve seat that needs to be replaced.
Either one involves taking the carb apart.