On most Unix systems you're likely to encounter, you can create a new
user by doing this as root ("#" is the prompt):

  # useradd jrandom
  # mkdir /home/jrandom
  # chown -R jrandom.jrandom /home/jrandom
  # passwd jrandom
    [...enter a new password, which may be a temporary password...]
  # su - jrandom
  $ chsh -s /bin/bash    <-- sets default shell to, e.g., bash
  $ exit
  # 

See also:

  http://svn.red-bean.com/repos/main/3bits/enabling_ssh_access_3bits.txt
  http://svn.red-bean.com/repos/main/3bits/new_sudoer_3bits.txt