The most simple way to change volume is to hover over the status text and scroll with the mouse wheel. To only use the keyboard you can either use the media keys if you have those or the p-assist, that means opening up instantASSIST using Super + a and typing in p. This brings up a volume slider. You can use the arrow keys, the j/k keys or +/- to change volume now. You can also use the mouse to drag or click on the slider. To set it to a specific value, type in the number and confirm with the return key. To close the slider, press any button that is not in use by the slider already or right click on it. Middle click on it to reset it to the value that is was at when opening it up.
You can also set the volume of individual applications. To do this, Super + Shift + Click on the status text or go into the Sound section of the control panel. Here you can set the volume for each application with a separate slider.
The settings to change input/output devices can also be found here.