Should I have to apply for signature campaign or I have to acquire more knowledge by improving my posts and rank as well.
There are some projects/ICOs that are accepting jr members.
What are the responsibilities/tasks that have to be done after joining signature campaign
It depends on you if you want to expand your knowledge of Bitcoin yet.
When you join a campaign you should know what to do, what are the rules. In my signature campaign we have 15 posts per week no spam posts and should not violate the rules. When the campaign is over and the project is successful, you will be paid.