Activity and rank come shortly after you read the sticky -> https://bitcointalksearch.org/topic/m.5106206
Trust comes when someone leaves you a rating. Different people leave ratings for different reasons thus there is no sure answer for that. Most involve some sort of trade though.
things. Threads get started constantly asking about when people rank up. It's all there.
As for trust, that's up to you and if you do deals here or if you're helpful to the forum. It does
not come automatically, i.e., you have to earn it. Don't be too anxious about it and I would
suggest not seeking trust actively. It arouses suspicion in the community and you could
even get tagged for it.
I would also suggest locking the thread if you've gotten your answer.