When was katniss picked for Mockingjay and who knew before she did?

I believe the "rebels" already were assembled or at least were connected and were just waiting for someone the people could connect with for a cause. When "they" (the rebels) saw how Katniss bravely vol. for her sister in the reaping they knew she was the one. That's why, I believe, Cinna proclaimed her as "The girl on fire" with his fashion designs for her to wear. She continued to gain the rebels respect with every "rebellious" thing she did, (shooting the apple out of the pig's mouth, and taking out the berries, etc.) They could see the fire in her and flamed it to ignite the people.

Well, the hints start in Catching Fire. Cinna knows, because he designs her outfits. And I wouldn't call it hints, since they actually say everyone but Peeta and Katniss knew about the rebellion(who were victors). Pretty much all the tributes in the quarter quell. Plutarch was a double agent in hunger games.Snow knew. Haymitch. Rebellions don't happen over night. They take time to build up and gain the proper defences so that you stand a chance in winning.

It started with Rue's death, but there was no uprising, just unhappy talk and unease. It was when Katniss and Peeta took those berries and were both crowned victors did uprisings start. It is quite vague.

she was actually mockingjay int he 3rd book but some decides when she did the knightlock and the putrack or how ev his name is knew like a lot knew ur question is not very specific so neither is my answer sorry