I actually started to join signature campaign when I was a jr member. That time they accept jr members. I don't know what it the rules now. But I guess it is difficult now to find bounties that accept jr members as they reserved it more for higher ranks. But well, I know you can find some. You just have to search more in the bounty section. Read more to know more. Read whatever requirements the bounty required you to do. Answer google forms and such. Putting signature codes. You can find some explanations regarding this on the beginners section. But all in all, you just have to follow the rules.