0755 is the octal notation of the permissions.
And the first 0 means that there is no sticky-bit set. If you'd want to set the sticky-bit, you would have to add 1 to the first number,  so 1755(in octal notation only; in decimal, you'd do chmod +t).

Whether a file is a directory, socket, etc.. is seen with ls -l at the first character of the permissions (d, s, ..), but they are not set using the first digit of the octal notation