Step 1: Channels
The most important aspect of understanding mixers is understanding the channels. On almost all consoles, the channels are laid out in strips; the signal comes in physically through the back of the device, then passes through that channel's various controls from top to bottom, with the gain or trim at the top and the fader at the bottom. I will go through what each of these steps does in their own, well, steps.
Simply look at one of the black circle inputs (XLR's for microphones and snakes), then follow the column of knobs straight down. This is a channel strip.
Step 2: From the top: Gain (Trim, Sens)