Every bounty has a different rule, you need to check if you are fit for their requirements or not.
Like, if they've allowed newbies to enter their campaigns, you can do the needful (as asked in their thread) and you will receive your stakes.
For signature campaigns, I can hardly find any that may be allowing newbies to join them as they look for higher ranks mostly.
For things like article writing and translation, you need to show your experience and you may get the job.
https://bitcointalk.org/index.php?board=238.0 <- That's the link to Bounties (Altcoins) board. Check it.