One of the biggest problems / key points when people are team building is that they don't know where to begin. Should I start with one Pokemon, a core, or a central idea? Everyone has their own way of team building, but here are just some of the basic ideas, or "frameworks", in team building.
Before any player can actually build a team, you should have a theory of how many roles / jobs that your team has.
Here are some team building structures I follow:
Note: This is just what I use when making a team. I suggest tweaking around the roles to see what you need the most.
Offensive Lead / Special Wallbreaker / Physical Wallbreaker / Sweeper / Revenge Killer or Scarfed / Offense Utility (it may be an Emergency Cleric / Spin Blocker / Pivot) or Spinner or Defogger
These teams are particularly considered about smashing certain checks / counters, so that the Sweeper or Revenge Killer can clean-up the opponent's teams.
Special Sweeper / Physical Sweeper / Special Wall / Physical Wall / Lead / Utility or Cleric
Balance can run a more offensive or defensive edge, but remember that you're not trying to wear down your opponent with residual damage or overwhelm them with offense. The main goal is to do somewhat a combination of both.
Cleric (with Heal Bell and Wish) / Rapid Spinner / Spiker / Phazer / Physical Wall / Special Wall
Stall teams really need to stack hazards and deal residual damage. For this reason, they often have recovery moves and status moves, such as Toxic and Will-O-Wisp (sometimes Thunder Wave as well).
Look here for further reference: Team Building Structures
The most important aspect of team building is to have good synergy. And I'm not just talking about type synergy. If your Pokemon don't work well together, but your overall team has no type weakness, your team would still fall apart. Each Pokemon must work to help achieve the same goal. Try to view your team as one working unit, instead of 6 different Pokemon with different roles.
Example: You're building an NU team around a wall-breaker... let's say Electivire. To make sure that any Physical Walls cannot stop Electivire, I would put a Special Wallbreaker, like Magmortar. Both of them can deal with each other's checks. Now, the only problem here is that they compound a weakness to Ground. I also need an emergency cleric to support Electivire and Magmortar. For this reason, I would choose Mesprit, which can provide SR + Healing Wish support as well have an immunity to Ground (Levitate) and has Energy Ball to deal with Ground-types in the tier (ex: Rhydon and Quagsire).
Finding the Best Pokemon
You should know your options when building a team. There are Pokemon that are threats and potential options for your team. Really, it is up for you to decide and test what works best on your team.
Anyways, that's it. I hope I helped!
Note: I may be making more edits later on.