I thought I'd add one thing: The very outer ring has a filter adjustment on it. Move the black triangle thing to cover the red marking in order to set it to neutral (no adjustment for filter), and then obviously point it to other settings if you are using a filter.
To do a battery test (since it's second-hand), there's a sliding thingy on the back. When you slide that all the way to one side, the red needle on the front should fly to one side of the meter (left, I believe). It takes px13 and px625 battieries for replacement (although mercury are no longer available in the US, though your username indicates Phillippines).