1. Choose Pokémon that fit in with your team's strategy – For example, say your team's strategy is to utilize the move Trick Room, which makes slower Pokémon move first and faster Pokémon move last. First, you'll want to choose a couple Pokémon that can learn the move Trick Room. You'll then want to choose slower Pokémon with either high Attack or high Special Attack to fill out the rest of your team. Using a fast Pokémon like Jolteon or Aerodactyl is counterintuitive to a Trick Room strategy since they will be the Pokémon attacking last when Trick Room is in effect. Whether your team's strategy is Trick Room, Rain, Dance, Sunny Day, or one of the many other possible strategies, make sure your chosen Pokémon fit within your team's strategy.
2. Choose Pokémon who don't share common weaknesses – Even though Pokémon like Zapdos, Garchomp, and Latios are all really strong Pokémon, if you create a team with all of them you will have three Pokémon that all share an Ice-type weakness. Even if your other three Pokémon are strong against Ice-types, it's too risky to use multiple Pokémon with the same weakness. I recommend having no more than two Pokémon on the same team that share a common weakness.
3. Make good use of your four moves – Each Pokémon only gets to use four moves in Battle, so you need to make good use of them. I recommend not using two Attacks of the same type, for example, you wouldn't want to use both Fire Blast and Flamethrower. Unlike when you're playing through the game, when you're Battling you don't need to have Pokémon that know all of the Hidden Machines or HM's, so using moves like Strength, Cut, or Rock Climb is a wasted move in my opinion.
Also, moves that are the same type as the Pokémon using it get a power up, so always try to use a move that's the same type as the user, even if it doesn't hit anything super effective like Normal. Try to fit moves on your Pokémon that can hit your opponent's Pokémon's weaknesses super effectively. For example, a Water-type makes great use of the move Ice Beam because it can hit Grass-types super effectively.
4. Use Pokémon that are able to beat popularly used Pokémon and strategies – The more powerful Pokémon are naturally used in Battle by more players. Make sure your team has a way to defeat these popular Pokémon because you will end up playing against them frequently. Have a way to defeat popularly used strategies, too. For example, it's always a good idea to have one slow Pokémon that can be used against opposing Trick Room teams, or a Pokémon that resists Water for Battling against Rain Dance teams.
5. Practice with your team and fix any weaknesses – Pokémon Black Version and Pokémon White Version introduced a way to battle against trainers from all over the world. Take advantage of this opportunity and battle against a lot of other people to see how well your team performs against various Pokémon and strategies. Just because your team looks good on paper doesn't mean it'll perform well in Battle. If your team is losing against the competition take note of which Pokémon aren't working well and figure out which Pokémon you can replace to fix your team's weaknesses. It's very rare for someone to build a really successful team on their first try, so don't get discouraged if you're losing a lot at first. Figure out your team's weaknesses, fix them, and keep Battling!