So, I'm also new, but I'll share a tip I learned from Diver's Ready that cut my SAC down by a chunk. Count while you breath. On inhale, do a slow 1-2-3-4-5-6, then exhale 1-2-3-4-5-6. Obviously there will be times you'll have to breath faster cause you're exerting yourself, but in my experience, most of my breathing underwater was just habitual breathing, not necessary for what I was doing, and largely in response to the whole "don't hold your breath" thing. So, yeah, count off as you inhale and exhale, and that should extend the time you get on each lungful without holding your breath (which obviously you should not do, AGE is no joke).
Also, if your reg has a flow rate adjuster or a predive switch, try turning it down? My reg (tusa rs1001) on dive mode breaths like a CPAP machine, and the guy at the shop where I bought it advised me to dive it in predive mode. Works much better that way.