You access the directory based on its path.
If you have a directory called "Music" in your home directory, you get there by typing "cd Music" if you're already in your home directory (which is where Terminal puts you by default)
You can also use "cd ~/Music" if you're somewhere other than your home directory.
If you're trying to do all this as the root user, you need to use the absolute path, e.g "cd /Users/username/Music"
If your issue is that you're trying to access a directory that has a space in its name, you tell the UNIX shell to avoid interpreting the space by using a backslash before the offending character. In your example, you would type "cd My\ Folder" (note the backslash before the space before the word "Folder".