You sound like a beginner so this will help you. :)
There's many many ways to build a team. There's now way over 700 Pokémon. Say a 1/3 are fully evolved, viable Eviolite NFE, different Formes, Mega Evolutions, and thats 230 (roughly) Pokémon you have to cover in just 6 Pokémon, 6 Abilities (7 with Mega Stone), 6 items and 24 moves. Its not possible. You've got to focus on something, a strategy, and play using that. Because a team can't do everything, either. It can't cover everything in just 3 Pokémon any more. Competitive team building is about making the most of what you've got, while covering as many weaknesses as possible. By focusing on one idea, one concept, you've got to try to get that to work, and use that to win. Instead of 6 Pokémon, you have 1 team, one goal in mind, and the game is instead getting your team in a situation where you can win. If it happens through luck before you reach your goal, then take it. If you consistently can't pull your strategy off, then you need to look at where its going wrong, and maybe there's a better way of doing it. If everything has already been done, then there is nothing original any more.
The best ingame team to add is: Sceptile, Linoone, Tentacruel, magnezone, Swellow and frosslass.
Hope I helped! :D