Go here: http://pokemonshowdown.com/
Then select a server. If it disconnects, try a different server.
OU stands for Overused. Each format(tier) has some Pokemon in it. Tiers are used to prevent something like a Magikarp VS and Arceus. If you're battling OU, it means that all the Pokemon in the OU tier are available for you to use in your team. You may also use Pokemon from all tiers "below" OU("weaker" than OU). The tiers are as follows:
If you battle in a tier, you may use Pokemon from a tier below it, but not above it. So, if you are playing UU, you can use Pokemon from RU and NU, but not from OU or Ubers. You generally want to make a team with Pokemon from the tier you are playing, and only a few from lower tiers.