The Details of the starting date are currently unknown at the moment. For most Tournaments, the LV Cap is 50, which includes this one. All you need to participate is a Pokémon Trainer Club account and a copy of either game with its Game Sync ID registered at the Pokémon Global Link. To actually register, you need to go to the Global Battle Union and register at Competition Info. Once done, you need to save and register your team in the Battle Box in game and connect via the Battle Competition.
Registrations start March 15th according to This Link. Also check that Link for additional questions you might have before asking here. Hope this helps.