When you join a ranked queue, it is draft.
Each team bans 3 champions so that neither team can use them.
After that stage each summoners picks a champ by himself and sometimes 2 people may pick at once.
There are no mirror matches, you can "counter-pick" as you can see the champion they locked in.
It is important you have at least some knowledge of each role as you choose your role in pick order.
First pick means you choose the first role, second pick chooses the second role and so on.
People normally state their preferred roles so they know which role they are stronger at.
You can sometimes ask nicely if you could have the specified role you want,
but not all players will let you.
Ranked is optional and you do not have to play it.