See if this works for you...
There should be a big knob reading METAL and MINERAL with an "O" in between.
Turn it so that the pointer is centered on the "O". It should be quiet there; that's called the "null." If it's not, move it a little one way or the other until it does get quiet.
Next, slowly turn it slightly toward the METAL side. As soon as you hear a faint sound, kind of like the buzzing of a bee, stop moving the knob. That faint sound is called the "threshold tone."
Now lower the searchcoil to the ground. The detector will probably go quiet when you do. Lift the coil and see if the faint buzz is still there. If it is, fine; if not, move the knob a little farther toward METAL until you hear the buzz again.
Pass the searchcoil over a coin on the ground. The detector should make a loud Beep! when it is centered over the coin.
Every so often, check to see if the threshold tone (buzz) is still there. It will gradually drift into silence or get louder as you use the detector, and when it does you will need to adjust it back to the threshold before you continue hunting.