Once you've decided on your team composition, the next thing you'll have to decide on are strategies. These are just generic and basic strategies. For more advanced strategies and other useful competitive tips, I recommend visiting Smogon University for that. .
Be reminded however, that things won't always go according to what you want or what you planned for. It is important to be flexible and let your own instincts and knowledge take over should a different situation arises. The Pokemon competitive scene offers almost endless scenarios. This makes it both exciting and challenging.
1. Create a team that will compliment each other's abilities and movesets
This is very important mostly for double, triple or sometimes single or rotation battles. If you just put together your favorite Pokemon in a party just for the heck of it, don't expect to win that often. Creating a competitive team requires careful planning - from deciding what Pokemon you'll include to your team, Breeding them to get the preferred IV, Nature or ability, EV-training their stats and even the items they'll equip.
For example, when a Politoad with Drizzle ability (has the similar effects of Rain Dance) enters the battle, the weather effect activates immediately. See the effects of Drizzle/ Rain Dance below:
For double battles, a good partner you can have is Heliolisk with Dry Skin. Not only will this allow it to recover a bit of HP per turn, it can also enjoy a 100% accuracy when using Thunder (normally has a 70% hit rate). Not only that, Politoed can use Surf to deal damage to all enemies AND recover Heliolisk's HP at the same time.
Another good alternative to Heliolisk for the example above are Pokemon with the Swift Swim ability. While Drizzle/Rain Dance is active, their speed will be doubled. The popular choice for this setup is Kingdra. With its Water/Dragon type, it will conveniently receive neutral damage against grass and electric attacks - the banes of water-types. For triple battles, you can have both a Dry Skin and Swift Swim Pokemon to exploit the advantages of Drizzle/Rain Dance.
2: Diversify!
Don't just stick to one type (or your favorite type alone). You'll end up having a team with shared weaknesses; expect to be mowed down quickly if ever your team faces one or two Pokemon in the opposing team that can exploit your weaknesses. Considering Strategy #1, you have to mix and match your team composition so that they won't hurt each other as well.
For example, Earthquake and Surf are some of the preferred offensive moves because they can hit multiple Pokemon in double or triple battles. You can't just carelessly use them if the other member(s) of your team is/are weak against them.
3: Make the most of your move sets!
A Pokemon can only learn up to four moves at a time. That's why role assignment should be considered before deciding what your Pokemon must or must not learn. The Move Deleter and Move Reminder NPCs in Dendemile Town gives you the convenience of deleting or restoring forgotten moves. You can also use TMs and HMs to give variety to your Pokemon's move sets.
It's a bad idea to have two offensive attacks of the same type. (Like Thunder or Thunderbolt). You don't want some unnecessary HM moves like Strength or Cut in your competitive Pokemon's move pool as well. If your Pokemon is a dual-type, include the most powerful moves of each type so that Pokemon can enjoy STAB damage for both attacks. Will a specific Pokemon act as a tank or staller? Protect is a good option. Will a Pokemon act as support? Helping Hand or Heal Pulse might be a good consideration to that Pokemon's move pool.
There's a lot of Pokemon and possible move sets to choose from. Don't be afraid to experiment until you get the combination you're most comfortable with.
4: Practice, practice, practice!
pokemon x/Y has the Battle Institute and Battle Maison where players can fight NPCs in battles normally available to competitive battles as well. Here you can fine tune your team, make adjustments and try out new Pokemon and strategies. Fighting human trainers are a bit different of course, since they'll have access to even better strategies, Pokemon and Teams. Keep in mind that even if your team looks good in simulation or paper, it doesn't mean they'll perform the same in a real battle. Don't get wary about loses; charge these to experience and adjust your team and strategies.