Find a bounty campaign somewhere in Marketplace . check if the bounty accepts jr. If not then find another, if yes then you may join. Put the right signature in your profile, some bounty do want you to post in some threads so you must check the rules coz it is included there. Most threads you're allowed to post is bitcoin discussion, economy, speculation, trading discussion, gambling, altcoin discussion, serious discussion sometimes Local. Take note that there is also a number of post required every week.