If you want your ubuntu behave as a NAS (Network Access Storage). You can use application called Samba.

First, you need install and config it by:

$ sudo apt-get install samba $ sudo nano /etc/samba/smb.conf

add this lines on the opened file:

[shared] comment = "shared folder" path = /home/ubuntu/shared/

And now add samba user with:

$ sudo nano /etc/samba/smbusers

add shared_user = "shared_user"