Your SMB username and password are the credentials that you use when you normally log in to your computer.
VLC on the Apple TV, using the Local Network tab, should already discover any SMB shares on your home network. Select one.
You need to have set up what to share on the computer. On Macs, turn on File Sharing in System Preferences﹥Sharing﹥. Then specify which folders should be shared. Other parts of your computer will not be shared through SMB.
You can use Anonymous Login in VLC if you have set any folders for sharing with Everyone in System Preferences﹥Sharing﹥File Sharing. Using Anonymous Login would skip the requirements for credentials.
Additionally, your Security and Privacy﹥Firewall settings on the computer should allow incoming connections for File Sharing.