The top bar of ubuntu on the right hand side there should be an icon
of two monitors (one overlaps the other).
If you right click on that you should have 'enable networking' and
'enable wireless' ticked.
Left clicking on the icon should show the wifi networks in the area
amongst other things.

If you don't see the wifi networks then that means that ubuntu does
not come with the drivers for your usb wifi adapter probably because
the drivers are proprietary.