I'd find someone on default trust to trade with. Once businesses is conducted, leave each other positive feedback.
Don't trade purely for the sake of gaining trust. That only leads to the opposite of what you want - people might look down on you for trust farming, which is pretty highly discouraged. But then you might ask, "how do i get green trust then?" and there really isn't a true answer to this. You most likely won't get it if you want it (without a ton of work and dedication to the forum and community as a whole) by just spam-trading. It takes time... and it's likely that you'll rack up a cool collection of red trust as well (untrusted, of course, by scammers and such).
and also how to give a feedback?
give them what they need so that you will earn some trust from them the more trust you get the more people in bitcoin talk trust you and you can easily apply on once's campaign for that matter you will not get rejected on a campaign but just be careful some people here is kinda weird they report you with no reason >.< 
