I strongly recommend, if you've never loaded shotsgells before, to get together with someone in the clubs that you shoot, who is experienced. Shotgun loads are usually pretty forgiving, but they can sometimes be catastrophic, splitting barrels, separating barrels, or blowing out the sides of chambers or barrels and causing injuries. Matching correct powder charge to wad column to shot charge is important, if you want consistent performance. For example, recently I saw a shooter using too high a wad column, that left a hole in the center of the crimps. In competition, a few shot ran out of the hole and jambed his side-by- side when he next tried to close it. Stuff happens, and the best way to avoid it is to know what you are doing ---and follow through doing it.
Here are a couple loads that I use in CAS competition. I use the heavier 1-1/8 oz loads for stiff knockdowns at distance, or for aerial clay birds.
12gn Alliant Extralite with 7/8 oz 7-1/2 shot, using Remington STS green or gold hulls with Claybuster Wad #CB0178-13
12 ga.
15gn Red Dot with 1-1/8oz of 7-1/2 shot in the above case with Claybuster Wad #CB2118-12
12 ga.
I'm in California and, surprisingly, I have been able to find both of the above propellants often enough to keep me shooting. But I scan local stores and online sources frequently and buy whenever I find.