There is a lot to know, and it's going to be a wall of text. You can check threads in this same subforum for more accurate info about some specific decks.
In general terms:
Start thinking 15-30-15 as a build rule, even though 14-34-12 would be closer to an awesome balance, Pokémon-Trainers-Energies.
If you need something (to make the deck work), add more than one, because if prized, you're lost.
You need a Hitter, usually high damage attacks with low energy, high HP. Mainly GX or EX. Add 3-4.
Then you need some partners to help the hitter (increasing the damage or the HP, completing the attacks with Special conditions, searching the deck for what you need... whatever). Example would be Garchomp+Lucario, Octillery, Dawn wings Necrozma for Golisopod GX, the omnipresent Zoroark-GX... Add enough (2-2 line, 3-3 line, 2 or 3 if basics).
But the most important part of a deck, are the trainers. Will get you the card you need, in the moment you want it. And that's why there are at least, double of the others.
The present Standard meta runs a trainers core kind of:
2 Brigette if the deck demands her (Tapu Lele GX combo for 1st turn set up)
1 Acerola if fits the deck
The most used items right now are
1-2 Float stone
2-3 Choice band
4 Puzzle of time
1-2 Field blower
3-4 Max elixir if fits
1-2 Enhanced hammer
Rescue stretcher or Super rod are also widely used. Evosoda became important as a lot of GX are Stage 1. And Fighting fury belt for basics, Also Nest ball or whatever fits in the deck (Rare candy, Switch, Prof letter, Energy loto, Energy switch, Multi switch...)
For expanded it's slightly different as you can include cards from 17 more sets which I think it's around 2000 more possible cards.
Finally, the minimum number of energies to make ir work.
That's how they look the competitive decks in a wide general term.
You can check most of the competitive decks checking the Regional results, both for Standard and Expanded, in here:
It's nothing wrong to copy a good deck or to use it as skeleton to build your own according to your playstyle or your budget of cards. At the end, everyone wants to win more than loose of course.
For any detailed information in deep of any trainer or any specific deck strategy or addition or change, don't hesitate to ask. Everyone will try to help.
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