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
shared_user = "shared_user"