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Team building is something that you kind of just learn as you learn about pokemon as a whole. The better you understand the tiers meta the better you will understand team building.
Just try to make "balance" teams on your own. A balance team is a team that just covers most weaknesses, so if one of your pokemon is weak to something you put a pokemon on that can cover that. a fire/water/grass core on your team makes team building a lot more simple.
Remember, playing on ladder isn't a big deal, it gets reset and it isn't hard to just switch names/use a different account for testing teams. People aren't going to remember the battles they have with you either, so there's no need for anxiety.
Just fuck around with shit, if you have troubles make simple cores like fire/water/grass.