This goes back to the basics. You should check Slant's and Skitty's guide which is pinned in this very subforum.
Make sure you use pokemon you like, depending on the tier. Then focus on offensive and defensive synergy. The easiest team archetype to use is balanced, so go for that now that you begin your career in competitive battling. Try to find roles for your pokemon. A team is not just 6 individually strong pokemon grouped together. Good things to have when making a team are:
1) A hazard controller - this pokemon puts hazards on the field or removes them from your side or both sides, or both. Essential in most teams, as chip damage can be proved detrimental for the outcome of the battle mid-game. It's also harsh to see your fire, bug, ice, flying pokemon to lose huge chunks of their HP from Stealth Rock, so Rapid Spinners or Defoggers are your best bet. A pokemon with Magic Bounce can also fit the role, but if rocks make it to your side, magic bounce cannot account for much.
2) A WALLBREAKER - this pokemon has the ability to dish huge damage, often without setting up. Great to deal with Walls that focus on outstalling you, getting your life bit by bit, while healing themselves with Recover, Softboiled, Synthesis or even with Wish (dangerous as it can be passed around).
3) A Wall - this guy can be physical, special or both. It focuses on stopping the opponent's attacks on one spectrum at least, so you can stop their momentum and build yours. Remember that when playing, you want to establish your rules on the field and make the opponent play your game, not the other way around.
4) The Speed - if not playing stall, this one is in every single team. It's a pokemon that either has great base speed, has priority, or is scarfed. Works if you need something that got too fast too soon gone, or to scout for moves. Usually those carry U-turn or Volt-Switch, to be able to deal some damage, while almost certain that their speed guarantees they hit first, and go away so something bulkier can take the hit. Hit-N-Run.
More roles are there. You might want to check smogon's lists of roles too.
What I like to have in teams that are more balanced is 1 or 2 pokemon that work as support for the rest of the team, crippling the major threats and allowing the sweepers and wallbreakers to do their job without much resistance. Usually paired with a pokemon that can set up in terms of speed.