Hi Andre, I split your post into a new topic as it may be helpful to others.
My advice would be to read the manual inside and out and watch the DVD that came with it, if yours didnt come with a DVD check out YouTube as Garrett have posted a bunch of instructional videos for the AT-GOLD. Between 50-100 hours seems typical to learn a new machine so dont get discouraged if youre not picking it up right away, just keep at it and you'll master it in no time.
Learn how to ground balance properly and do it often, changes in the ground composition can impact the performance and depth of the detector.
Try each mode in different ground conditions to work out which mode works best in what situation. I personally try to use All Metal Mode wherever possible as this mode provides the most depth but you will lose tone ID and the target ID accuracy is supposedly effected.
In time you will learn the different characteristic of common targets. For example on the AT-GOLD, a modern pull tab in All Metal Mode shows up with a target ID of 54-55 while a Gatorade seal shows up as a 42. Knowing this information can help to answer the "to dig or not to dig question", although I highly recommend digging all targets until you get used to the machine.
It's not always practical to use All Metal Mode, for example in trashy areas I use Discrim 1 which helps quieten it down a little, Discrim 1 also provides tone ID which can be very useful for identifying quality targets. You may also want to use Discrim 1 when the detector is being effected by hot ground or hot rocks, ultimately you would want to at least try to use All Metal Mode when searching for gold as this is the preferred mode for maximum depth and sensitivity. I almost never use Discrim 2 but you could once again use this in trashy areas or when searching specifically for coins.
Practice pinpointing, once again the manual or YouTube might be the best place to learn how to do this.
One last thing, don't detect with Iron Audio turned on, only turn it on to test a target then switch it off. Iron Audio combined with other target indicators can help identify trash metal, BUT be aware that Iron Audio can falsely sound on a quality target, this could be due to the angle of the target in the ground, if corrosion is present or if the quality target is sitting right next to trash metal.
Swing low and slow and just have fun with it. I hope everything made sence, its late