Pokémon Showdown is the most common Battling Simulator with competitive battling - essentially what you do there is create whatever team you want, and then click 'Look for a Battle' in the correct tier.
Some things that you should understand are;
Showdown has a list of Pokemon in tiers, if you check in the teambuilding. Use this to create your team. The tier of OU (Overused) is the most common tier - and to play in that tier you can only use Pokémon in the 'OU Tier' or in a tier under.
EVs, IVs and Natures all increase or decrease a certain stat on a Pokémon depending on the 'investment'. Read the links above for more info.
If you're battling over Wifi, you also need to know about EV and IV Training/Breeding. This is so you can compete with other people who would otherwise have Pokémon who have superior stats and a beneficial nature.
This site has some tips on EV Training here
You can also find a lot of advice on stuff like this over previous questions on the site - eg Fondant's answer here