If you are newbie then you need to keep posting, and then one day you will reach a higher rank. When you have a higher rank you can join signature bounty.
But there are lots of different bounties, you can do twitter, blog, translation or video.
I agree, eventhough you are a newbie it doesn't mean you can't join in bounties. There's a lot of bounties that you can join in your rank. Like twitter and Facebook campaign. You can join also in telegram. You just choice a bounty that accept a newbie.