I have a Garrett GTI2500. It is my first and only detector so far and I have had it for a little over 4 years. I feel the best way to get used to your detector is to get it outside and practice. I never set up a test garden (although if you can I would recommend it). Instead I just started swinging and digging. As Warsawdaddy said read the manual and if you have it, watch the video. Take your time. Initially you will probably dig a lot of trash but in time the goodies will start to appear and with more frequency as you become more familiar with your machine. I mostly keep mine in "jewelry mode" since I generally hunt parks and schools. Keep in mind that despite the claim the discrimination is like "seeing in the ground" no machine is foolproof. The "depth" and "size" are usually pretty accurate although they too can be quirky sometimes. The pinpointing, if used appropriately is also fairly precise although multiple targets below the coil, the "halo affect" some targets give off and in my case sheer laziness or impatience can leave a nice gouge or scratch (cringe) on an otherwise lovely coin. The GTI2500 is an overall sensitive and rugged machine but is only as good as it's user. The more time you spend with it the better you will get. Good luck and HH.
P.S. Most of the gold rings I have found have read in between "nickle" and a couple notches above "pulltab".
Also, welcome to the hobby and this forum. If you take the time to read, there is a wealth of information on this forum that will help you go from a beginner to a seasoned pro in much less time than you would otherwise on your own.