Competitive teams can be hard to build, and this is just a few rules that can help you when building a competitive team.
1) Make sure your team has a goal in mind. Successful teams all have a general strategy that all the individual pokemon help with. This goal can be something like stall, offence, bulky wish passing, or even an entire team built around setting up a single sweeper. Having six OU pokemon with no central strategy doesn’t make for a good team, it just gives your opponent easy wins. A few show guides of teams will be listed below, to help if you’re making one of those:
Offensive teams are the most basic style of teams that you will encounter. It takes down opponents teams by simply attacking the opponent as hard and as fast as possible. An offensive team will usually have six powerful sweepers (choice, setup, and kind) that can work well together, and between the six of them they can kill anything in the game. The only residual damage that an offensive team will nearly always have will be stealth rock, and sand if you choose to use Tyranitar.
Because offensive teams will lack bulk, speed and priority are important. Not everything can be countered with an offensive team, so you will occasionally need to rely on strong priority attackers (Lucario, Scizor, Mamo) or a fast Scarfer (Latias, Gengar) to get rid of things that set up against your team. It’s also usually a good idea to have a lategame sweeper that can setup and in a single turn to clean up after the other five sweepers have helped chip away at the other team (Salamence, Gyarados).
Stall is a form of play that is the most defensive play style you will ever encounter. Stall typically beats its opponent in one of two ways. One, residual damage such as entry hazards, and status in combination with wear and tear damage from defense pokemon attacks. Two, the opponent gets pissed off and leaves. The first thing you do when you decide to make a stall team is decide which six pokemon to use. Once you decide this you start off with picking your blissey move sets.
*Note: Blissey is a requirement for all stall teams. Since the other five pokemon will be used to counter physical / mixed threats, and support the team. It is also typically important to use a Ghost (Rotom , Spritomb) and Celebi. Celebi offers a means to win late game (Perish song) and also provides the team with much needed support. The ghost provides the team with rapid spin blocking most notably rotom provides the team with a gyarados counter, and a status absorber.
Balanced teams are, well, balanced. Balanced teams consist of bulky sweepers and tanks(occasionally a frailer sweeper or two), that have good synergy. They don’t need to rely on revenging nearly as much as an offensive team, and they can’t counter as much as a defensive team, they’re somewhere in the middle. Balanced teams, like all teams, nearly always have stealth rock. They also take advantage or status effects like thunderwave and WoW, while most stall teams use toxic for more passive damage, and offensive teams generally will have nothing outside of maybe a fast sleep.
2) Always check your weaknesses chart. Having 4/6’s of your team weak to fire, or electricity, or anything kind of sucks. A general rule is to have no more than two pokemon weak to one type of attack, or two always have at least at many resists as you do weaknesses. I like to follow that second rule, and it’s served me pretty well. A good tool to use is the Marriland Team Builder: