This part in my toilet slowly drips water:
It's not a big problem but I still want to fix it. I already tried to screw the part as tightly as possible but it did not change.
Should be an easy fix. Shutoff the water at the valve on the wall. Unscrew the leaking joint. Try cleaning or replacing the washer. And then reattach.
If that doesn't work, replace the hose. They are sold with the plumbing supplies with the other toilet parts. The only part that is throwing me off is the white plastic angled piece in the hose. So if you need that, and can't find an exact match in the hardware store, you're better off just fixing the washer.
Edit: looking at the picture again, I'm guessing the white plastic is the inside of the hose and the metal braiding is pulling back from the end. If that's the case, I'd replace the whole hose with something a little longer. I'm pretty sure they come in several lengths in the store, and you could use an extra 6" to make a better curve in the line.