So when i make bend lines to make mouth with pen tool in photoshop, when i stroke the path, it looks like that:

How do i make them "smooth"?

Heyyyy.
Check this tutorial specifically made for pen tool:
http://forum.toribash.com/showthread.php?t=282169

Oh and more useful art tutorials here:
http://forum.toribash.com/forumdisplay.php?f=510

Cheers.

For this sort of thing you can also use google. There are heaps of tuts out there for people such as yourself that want to learn.