Likely it's a loose washer in the shutoff valve or a loose seal in the
Neither are a big whoop to fix/replace if you know what you're about
with that sort of stuff.
There are plenty of "DIY toilet repair" websites, here's one:
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.