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
Mostly by joining campaign you will need to meet at least minimum post requirements like 10,15... Some of them have post restrictions like number of words, in which topics etc. you can find details on a particular 1st post from campaign as well as information how you could apply.
responsibilities usually for your posts to be constructive and if you meet minimal requirements you should be paid.