Teams are the backbone of pokemon. Firsty, you must learn about EV training and breeding. Learn about those before you see this.

Teams can have a theme ( Water, Wall, Tank, Sweep, etc ), or just have a format ( 3 Sweepers 3 Walls, 2 Spec Sweepers 2 Spec walls 2 tanks, etc ) but must have a balance. I will slowly build a team in this guide to get you started. First, people usually pick theme or format. This will be Format, because its simpler. 1 Mix Sweeper 2 Pys Sweeper 3 Walls. Usually, you first pick a leadoff. Our leadoff will be a ScarfCario.

Lucario@ChoiceScarf
Lonely/252 Spd/128 SAt/128 Atk
Aura Sphere
Iron Tail
Crunch
Dark Pulse

this is a decent lead off! Now, lets examine its weakneses, fight fire and ground. Now, we need to think of a pokemon that easily fends off these types. Salamence is a good choice.

Lucario ( see above )
Salamence@Life Orb
Lonely/252 Spd/178 Atk/ 80 SAt
Flamethrower
Dragon Dance
Dragon Claw
Earthquake

Luc also counters its Dragon and Ice weaknesses, so already we have a good pair of sweepers. Lets starts on walls. Walls are used to defend ur sweepers or usually set up a strategy. Baton Passers, although not always, are usually also walls. A very common Special Wall, Blissey, will be our example.

Lucario ( see above )
Salamence ( see above )
Blissey@Leftovers
Bold/252 Hp/252 Def/6 SAt
Softbroiled
Ice Beam
Calm Mind
Auromatherepy
"Serene Grace"

Blissey has excellent hp and spec defenses to wall. Auroma for team support and at the same time can counter dragons. Now, it got LOW defense, and is fight weak. Dusknoir can take care of this.

Lucario ( see above )
Salamence ( see above )
Blissey ( see above )
Dusknoir@Leftovers
Impish/252 Hp/252 Def/6 Atk
Shadow Sneak
Thunder Punch
Will-O-Wisp
Pain Split

its a great wall, can offend if it needs to, and counters gyara. Ownage. Now, because blissey IS common, we will need more physical stuff. Lets get a heavy hitter - Machamps good.

Lucario(see above )
Salamence ( see above )
Blissey (see above )
Dusknoir(see above )
Machamp@Lum Berry
Jolly/252 Atk/252 Spd/6 Hp
Dynamic Punch
Stone Edge
Pay Back
Earthquake
"No Guard"

Now this is starting to look own! With the mix support from Lucarion we have wall breakage, Mence is a nice bulky sweeper with flamethrower for Skarmory, Blissey absorbs special attacks like nothing, dusknoir is a beast, and machamp packs the punch! Its not done, but teams are supposed to look like this - balance in all three aspects - Defense, Offense, and Typing. The Last pokemon is usually going to either be a late game sweeper or a back up wall. In this case, a back up wall.

Lucario ( see above )
Salamence ( see above )
Blissey ( see above )
Dusknoir ( see above )
Machamp ( see above )
Bronzong@Leftovers
Relaxed/252 Hp/252 Def/6 Atk
Earthquake
Light Screen
Gyro Ball
Hypnosis
" Levitate "

There you have it! A great team, with balace in Offense, in Defense, and in Type Coverage. I hope this helped. NOTE: Of course, im not saying USE THIS TEAM!, im just trying to show how the building process should work. I would also like to point out, teams with a theme ( rhyme power ) usually build in the same way, unless its a type theme ( water, fire, etc ). In that case, trying using secound types to balance out weakness.