I solved my problem by this way:
Create a new group
$ sudo addgroup exchangefiles
Create the chroot directory
$ sudo mkdir /var/www/GroupFolder/
$ sudo chmod g+rx /var/www/GroupFolder/
Create the group-writable directory
$ sudo mkdir -p /var/www/GroupFolder/files/
$ sudo chmod g+rwx /var/www/GroupFolder/files/
Give them both to the new group
$ sudo chgrp -R exchangefiles /var/www/GroupFolder/
after that I went to
/etc/ssh/sshd_config and added to the end of the file:
Match Group exchangefiles
# Force the connection to use SFTP and chroot to the required directory.
# Disable tunneling, authentication agent, TCP and X11 forwarding.
Now I'm going to add new user with obama name to my group:
$ sudo adduser --ingroup exchangefiles obama
Now everything is done, so we need to restart the ssh service:
$ sudo service ssh restart
notice: the user now can't do any thing out
I mean all his file must be in file Folder.