I have been watching battles since X/Y came out, but I started battling when Nations started. To start teambuilding, you'll need to find a single pokemon or pair of pokemon to build around. They can be any of the roles listed below. Its quite easy to teambuild when you pick a pokemon capable of mega evolving. This is not mandatory to have a viable team keep in mind.
Then from there, you'll do well with support/utility. Such as pokemon that can set up entry hazards, heal others, etc. This role can be played by two pokemon if you like.
To keep from being walled, make a wall of your own. I typically run two of them for physical and special defense. Walls can also be supportive and set up hazards.
If walling a wall doesn't work, you'll need a wallbreaker. Wallbreakers do what they suggest. A great form of wallbreaking is using priority. For example, a choice banded Scizor can rip apart almost every fairy type wall, such as Sylveon and Clefable. Some great wallbreakers are Magnezone, Crawdaunt and Porygon Z.
Staller: If not taken care of quick, stallers will destroy your team. Some of the greatest stallers of the metagame are Chansey, Gliscor, and Shuckle. If you have problem dealing with some of these, grab a wallbreaker. Taunt is also a good way of shutting them down.
You knew I was getting to this, huh?
And there is the Broom. They also do what they suggest. Sweep. Bulky or not, they mean business. In order to do so, you'll need to take care of any walls that might get in their way.
A team can consist of any amount of pokemon that play any number of these roles. They just need to work well together, and if they do, you got yourself a nice team.