lemon wrote:
tell me how to use a USB connection on Ubuntu
Just guessing, you mean a mobile broadband USB dongle?
If so, then right-click on the Network Manager icon in the top-right panel (looks like four concentric quarter-circles) and select Edit connections. Open the Mobile broadband tab and click Add. With any luck, all you need to do is to select your USB dongle from the list, and it will be the only device in the list.
You should have the device plugged in before opening Network Manager, and possibly you need to boot up with the device plugged in - it is supposed to be auto-detected, but does not always work.