Fiberglass Repair Tips

If it is a shallow injury, limited to gel coat and perhaps underlying mat, you can probably get by sanding out loose material and using something like Marine Tex -- pretty easy to use, and can be formed and sanded easily.
Various polyester fillers are available, and fairly easy to use, with instructions on the can.  Finish with gel coat (you may find exact match already commercially available).

If it is more serious, I'd use epoxy resin. This is not difficult, but can be messy.  Check Gougeon Bros booklets available where Gougeon Brothers (West System) Epoxy products are sold.  If the damage is cosmetic, you can probably grind clean (I'd use 80 grit sand paper on an orbital sander or die grinder - I prefer an air powered die grinder upon which I use five and six inch adhesive sanding disks.  Works like a charm). Use a respirator when sanding.  Clean thoroughly before sanding, sand until you have a solid base
(any depressions should have a 12/1 slope when you are done sanding, so, for example, a 1/4 inch gouge 1 inch in diameter would have a sanded, dished area about 7 inches in diameter -- 3 inch radius, plus the 1 inch hole), then fill with thickened epoxy, such as West System 105 resin and 404 filler.  Use disposable gloves (I buy them in the hundred pack at discount stores for about ten bucks).  Also handy are the Popsicle sticks and disposable cups for mixing (no Styrofoam or wax cups), plastic squeegees (varying sizes), and plastic sheeting (polyethylene) for laying over plywood on saw tables for a re-useable work surface.  The poly makes preservation of your work table easy.

Don't mix too much resin at a time -- experiment with small amounts until you are comfortable with it.  Don't put more than a quarter cup in a plastic cup -- it will melt the cup -- and don't even put a quarter cup in if it will be in there long.  You want to use it before it begins to heat up.  If it feels hot, it's too late to use it and still get decent saturation (even if you CAN get in properly spread).  Be careful where you dispose of mixed epoxy, until it has set, cured, and cooled.  I have heard reports of fires started spontaneously -- but never had that problem myself.  Still, I toss unused, mixed epoxy aside (not in the trash) and pick up and dispose in the trash after I am done for the day, when the stuff is done reacting and has cooled.

If there is damage to the cloth or roving in the hull, you will need to grind down to solid material, then build it back up with cloth.  Many layers of thinner cloth is better than heavier layers.  West System manuals describe the procedure.  You can stack a series of patches of cloth cut with increasing diameters like a stack of pancakes, wet them all with fresh resin, then lay this in the wound.  It is critical that you work quickly to maximize the soak time -- the greatest reason people get blisters is due to the existence of less-than-ideally saturated fiberglass cloth fibers, allowing moisture migration.  This happens all the time, even in commercial production.  It is not enough that you get the stuff in place before the resin starts to gel -- you want it to lay there soaking long enough to fully saturate all fibers before it starts to set.  Once the stuff begins to stiffen, it will not continue to wet out the fibers; if they are sealed but not wetted, your patch will lack maximum strength, and the fibers will never be fully resistant to moisture penetration.  One wicking loose end, and voila! Eventually you'll see blisters. (If this repair is well above the waterline, you have less concern, as it will be less susceptible to moisture saturation such as cause blisters.)

After the epoxy cures, grind it smooth, fill with a good putty -- I mix mine from West System epoxy and fillers (above waterline, many use Formula 27 -- cheap and easy to use).  Finish with gel coat (you may need PVA to final cure the gel coat, but I have used Saran Wrap on small repairs -- gel coat needs to be deprived oxygen or it will remain tacky).  After the gel coat is set, sand lightly (perhaps best to do this by hand with a sanding block, at least on small areas), then buff to desired shine.