(edit******wow, first post and I'm bringing up a 3 month old thread.......sorry, but thought it useful.
First thing, turn off the fish ID feature and turn down the sensitivity some. You want it so your screen has a couple little dots on it once in a while. usually have mine (Lowrance or Eagle units) between 75 and 85% depending on depths, water temps, and water cleanliness. Almost every variable affects how the sound waves move through the water. Colder, dirtier, and shallower water lean me toward lower sensitivity since the water is more dense (colder) or has more particles (dirtier) so the screen will show more clutter on it. really shallow water, 5 ft or less, the sound will sometimes bounce off the bottom, then the surface, then the bottom again before the transducer picks it up, resulting in doubled depth readings. Turning down sensitivity helps with this sometimes.
If it's set up right, and most of the time there's nothing on the screen, when you see stuff marked on the screen, like lines, arches, or just blobs, trust that there's something there. Might be fish, bait, or grass floating, but there's usually something there if a unit that's set up right says it's there.
The FF can't show you where in the sonar cone a mark is, only how close to the transducer it came, and how long ago it went through the cone. Try to think of it as how far away from the transducer it is more than how deep it is. You might pass over something that's 15 ft to your side and it'll show on the cone as a blob or line. If it goes right under you, you get an arch.
Things to the left of the screen happened first, to the right is more recent.
School of bunker with bass under them. You can see how the bass marks under the bunker are slanted upward. That's fish that are moving up. If they slanted downward it's fish moving down. Remember that the right side is more recent.
Also, if you see some structure that looks like it should hold fish, just cause you don't get any marks doesn't mean there's no fish there. it only means your sonar cone didn't go over them or they're too tight to the bottom to see.
In this one, the red line is my lure. There's a school of crappie on the bottom. You can see on the left how the lure's at one depth, I lower it a little, then bring it up and down a couple times. The blue line is moving up, that's a fish coming to look at my bait. Where the line goes almost straight up is where I set the hook and raise the fish up, then he goes back down for a minute then leaves the cone.