how to discover all WiFi networks on my Ubuntu 15.10 and choose one of them?

Solution 1:

Use iwlist. The man page (actually http://manpages.ubuntu.com/manpages/wily/en/man8/iwlist.8.html) says:

wlist.8.gz
Provided by: wireless-tools_30~pre9-8ubuntu1_i386

NAME

       iwlist  -  Get  more  detailed  wireless  information  from  a wireless
       interface

SYNOPSIS

       iwlist [interface] scanning

snip...

PARAMETERS

       scan[ning]
              Give the list of Access Points and Ad-Hoc cells  in  range,  and
              optionally  a  whole  bunch  of  information  about them (ESSID,
              Quality, Frequency, Mode...). The type of  information  returned
              depends on what the card supports.
              Triggering  scanning  is  a privileged operation (root only) and
              normal users can only read left-over scan results.  By  default,
              the way scanning is done (the scope of the scan) is dependant on
              the card and card settings.
              This command takes optional arguments, however most drivers will
              ignore  those.  The  option essid is used to specify a scan on a
              specific ESSID. With  some  card/driver,  this  enables  to  see
              hidden  networks.  The  option  last does not trigger a scan and
              read left-over scan results.

"How do I connect to a wireless..." is a second question that should be asked by itself, in a different Question post.

Solution 2:

If you have a graphical environment installed, there should be a network indicator icon somewhere on the desktop, (most of the time in a bar on the top of the screen), and you should be able to find your network there in its dropdown menu. Click on the network indicator icon and check Enable Wi-Fi to show all available Wi-Fi networks. Click your network, fill in the password, and it should connect.