Fortunately Humminbird is simple but powerful. Start by scrolling through the menus. The first menu button brings up the short list of most likely options for the screen you are on. If you are in the chart, it brings up chart options. If you are on the fish finder, sensitivity and other sonar options come up. I find these very intuitive.
The next press of the menu button brings up all of the possible options. Other than one waypoint management screen There are no menus deeper than this. A key find is to change user from normal/standard to advanced which expands some of the menus.
From there it just takes time on the water, Catching a few fish, and I use a camera to confirm some readings too.
The best references for H-bird are—
Doug vahrenburg’s tips and tricks
The Technological Angler
Bruce Sampson has a great website too.
At Chatfield I put a waypoint on every drop off I saw and eventually the points, reefs, and roads started taking shape. Even after Fishing once a week for 4 years I was still finding gravel pits and working new spots.
Another good way to learn to trust the readings is to buy or set up an ice Fishing rig. It is easy to fiddle with the settings on the ice and the jigs and fish come thru clearly.