1.In short explanation, when a company wanted to advertise their website, they will open a signature campaign where they'll pay us to put their signature which will be seen by another users (see bottom of my post)

2.Well you need a good post quality and not a spammy one, check my post history for reference though my post quality isn't that good compared to other users here

3.No you don't, different companies will ask a different rank for your account, currently the lowest one is Jr. Member which accepted by Secondstrade

4.Legendary worth more than 0.1 BTC, probably 3 Bitcoins as they will require at least 3 years to make

You can check all of the current available campaign here
https://bitcointalk.org/index.php?topic=615953.0