Hi,
I recommend running in the All Metal Mode, all of the time and dig everything (at least for now). Dig all weak, repeatable signals. If the target does not have a repeatable sound, it could be a hot rock or a false signal.
If you decide to switch into the Discriminate mode, put it on 2.5....as high as 3 when looking for nuggets. If your think you have iron (in Disc mode), swing the coil quicker to see if you can cause a crackling/popping sound.
Keep your Sensitivity around 10, give or take 2. If you can go higher without hearing 'chatter' from the ground, then go as high as possible. If you have too much chatter, and have to turn your Sensitivity down below 8, then switch to the Alkali or Black sand mode (I prefer Alkali). This will help in very mineralized ground.
If you are looking for other items, coins, etc., then you will have to figure out where various items are discriminated out doing the air test, using the manual and in-the-field experience. The manual is a good start, Air Testing. If you have a fax, I will fax you a sheet with approximate setting for the Disc and various metals, foil, copper etc.
For Hot Rocks, pump the coil up and down 3 times over the suspected hot rock. If it disappears, you have a hot rock. If the signal does not disappear, dig it, not all hot rocks