I’ve played VGC a lot before, so I decided to answer.
Some tips for building VGC teams:
Do the Pokemon in your team go well together? VGC teams should have supporters and attackers, and often redirection and speed control are essential. Redirection (Ally Switch, Follow Me) are important for supporters. It can help an ally set up. Another example of redirection is abilities like Storm Drain and Volt Absorb. This is one reason why Togekiss is such a well used Pokemon, it gets Follow Me. Speed control (trick room, tailwind) are reasons to use Dusclops and Whimsicott respectively. As for Tyranitar, it looks cool. Only joking, it is used because of Sand Stream, which is weather control and is the core of sand teams that often also include Exadrill. That’s an example of a good team combo, Tyranitar and Exadrill. Their abilities go great together and they both highly benefit from Sand because of their typings. Another example of why a Pokemon goes well win another is Whimsicott and Dracovish. Whimsicott can use Tailwind to boost Dracovish’s speed to help with Fishous Rend. Their moves support each other.
When I build a VGC team, I often have an idea of it in my head or make a team based around a certain Pokemon. I get a Pokemon, make a set for it then look at its ability, weaknesses, EVs and moves and decide on a good ally for it. For example, the other day I was building a team around Inteleon, so I saw that it had low bulk and no Focus Sash, so I decided to run a Togekiss for Follow Me. Rotom Heat was my next Pokemon because it resists both of Inteleon’s weaknesses.
That’s pretty much it.
Hope I helped!