Many people have trouble when it comes to cutting sheet metal, particularly women and those with smaller wrists and hands. Of course, those with big hands and thick wrists will be able to exert greater pressure when cutting, but basically anyone should be able to cut 20-gauge (or thinner) sheet metal with relative ease.
The first thing you need to do is to purchase good-quality metal cutting pliers, most commonly called "tin snips." You need three of them: right-hand, left-hand and straight cutters. Generally all three come in color-coded sets (red handle is right-hand, green is left-hand and yellow is straight) and can be found wherever hand tools are sold.
What, then, is good quality? Well, if a metal cutting pliers is priced less than $10, the chances are it's not very good. The "action" will be stiff and the blades not of sufficient hardness to hold a good cutting edge, so cutting metal with them will be difficult. When looking for pliers, take a piece of sheet metal (ask a neighbor for a piece of a car fender, metal house siding, real estate sign, etc.) with you and try out the various brands.
Buy the pliers that are the easiest for you to use and that fit comfortably in your hand. Some pliers's grips spread so far apart that only very large hands can get around them, so avoid such designs. The better brands are designed with parallel grips and multiple pivot points, making their use much easier.
Cutting metal isn't like cutting paper or fabric. Unless the metal is thin, don't push the jaws of the cutter all the way and start compressing the handles. This will result in high effort and fatigue. Instead, insert the jaws about one-third the way and then cut, using the proper pliers for the direction of cut.
Mark the cutting line on the piece of metal to be cut, then make the cut on the outside of the line, leaving the line itself. The reason for this is that snips leave a rougher edge, so the leftover line allows for final filing or sanding for a smooth edge.