Basically I say there a few things that you need to understand to go into competitive. The first is breeding. There are many breeding guides you can find on youtube, but the idea is utilize the 2 items destiny knot and everstone to create perfect IV's. If you do not understand IV's you may want to do some more research before reading the rest of this answer.
Next, is EV's. EV training is very simple in SM and you just have to understand how to use Pokemon Pelago, which can be accessed with the drop down menu with the x button, and the same is with the IV's, if you don't understand EV's please do some more research.
Now, once your done EV training, and breeding you have to build your team. Pokemon DB has great simple type matchups and you can access this just by typing a type into the search menu. The idea is to create a team of 3, or 6 Pokemon that cover each other type wise, and have the least weaknesses, and the most coverage.
Finally you will need these on your teams: Special tank, physical tank, Sweeper/staller.
Here is an example:
Sweeper: Mega-scizor
Physcial tank: Gliscor
Special tank: Comfey
Scizor is very weak to fire, so I have a ground type, gliscor, to counter that, but becasue I have ground I can run a fairy type, comfey, because ground is super effective against poison and steel, which is super effective against fairy. But gliscor is very weak to Ice types, so that's why I have bullet punch on my Scizor, so I can take care of the ice. Comfey can wall basically anyhting else out.
i almost never lose with this team, even when I'm going against legendaries.
I hope I have answered your question, if you have any more questions please ask, and I will try my best to answer your question.
