As detailed in the battle thread, you will post your squad of 6 and send out a Pokemon after HnC posts his/her squad. Then HnC posts a Pokemon and attacks, then you'll attack. After each reffing, the attacking order will reverse.
For more information, you can consult the PASBL website. Give it a quick read-through since it's your first time, then keep it in mind if you have any further questions, and if the site can't answer, there's plenty of people (LOs, etc) you can consult as well.
I hope that helped.