If you're looking for cores, a good place to look is the archive of the
Good Cores thread. While atm it is still p. barren, it will keep increasing in size as the gen goes on and more and more cores are posted.
The definition of a core is a combination of Pokemon which work towards achieving a specific goal. For instance, a defensive core can aim to cover the weaknesses of it's partner, such as Skarmory+Chansey (Skarmory covers physical attackers that give Chansey a headache, Chansey covers special attackers that give Skarmory a headache) or the many type cores which aim to cover each others' type weaknesses. Offensive cores typically aim to either overwhelm shared checks (Landorus-T+Garchomp, Bisharp+Weavile etc.), to punish/take advantage of typical answers to the partner (Keldeo+Tyranitar, Special attacker+Screech Dugtrio etc.) or to help put one member of the core in a good position (Pelipper+Swift Swimmer, BP Scolipede+wallbreaker, Alolan Ninetales/Dual Screener+setup sweeper etc.). Just generally think of how they interact with their partners and design cores like that--that's the best advice I can give you.
As a general rule of thumb, you are going to have a harder time using Pokemon like Dhelmise which have little or no competitive viability. This is why the viability ranking is helpful--it gives you the jist of what is generally good/consistent and what generally isn't.
People are willing to help and keep helping if you're finding it hard, but only if you let them do so and are patient about it. You don't learn an entire game overnight, and getting frustrated over it isn't going to get you anywhere and is actually counterproductive. I'm going to repeat my suggestion of going to battling 101 'cause it is extremely useful on the whole and you will probably benefit heavily from it.