please watch some youtube videos first and educate yourself on how to use a wallet and send/receive before you sign up for any bounty...

Participating in bounty is easy but you need to maintain your reputation first on bitcointalk.
At least become a member then there will be no point.
After that goto Altcoin Announcement thread where you'll come across different cryptocurrencies being launched.
There will be a link provided for their bounty programs and announcements with proper instruction to participate and work.

Sample:- https://bt.irlbtc.com/view/2142876

Making money is not so hard from the bounty. You need to just choose a handsome reward paying the company and register for the same. Mostly but almost all of the company offering social media bounty which is very simple to register and earn. The work is very simple, you have to do make some like/share in facebook and tweet/ retweet in Twitter. If other available like; Linkedin and Reddit or Instagram then the work is same. But the hard part is that Translation bounty which will give you more reward. Basically, the company offering bounty pay reward as the token. You have to cash them when the token hit the exchange. Some of the popular exchanges are; Bittrex, Poloniex, Livecoin, Cryptopia, Binance etc. and still, some new exchanges are evolving frequently.

Your newbie no so first know about Bitcoin forum by watching Google and YouTube it only teach you how to earn money in bounty for more information check in forum only.

Hey. You can try in this Bounty TokenGo project. Participate can even Newbie. Plus I think the presence of a private office instead of all boring Google-table.
Learn more - https://bt.irlbtc.com/view/2497405.0

For participating in bounty you must have some skill. You must be able to advertise the project in one way or the other may it be from twitter or facebook or even using your signature. Once you complete the assigned task you will get tokens out of the assigned pool.

So let's say that we increase our rank here on the forum. But if we don't have a previous work as a translator for example, how we can catch a translation job? Because they ask for a previous translation.

First, you read complete terms condition of bounty and follow its rules then you earn form bounty

Ya it is true that you can earn good through participating in bounty programs, as when this coins hit exchange and if they are worthy then its price will be good to trade. You have nothing but keep watching all bounty programs and follow their rules. When you join one or two bounty you will come to know full details, In that announcement thread itself you will get all help from the community members, So their is no worry for it.

if u want to earn from Bounty u should be aware of news and happening to reply to quires and u should have minimum junior member rank and participate in icos