You're 2 years away from finishing a PhD in South Asian history (according to your other posts). Now you're applying for social sciences postdocs?
Try reading previous threads here for information about postdoc applications, and don't post duplicate questions.
Can someone direct me to any sort of resource that may help me to write my applications?
This resource is called YOUR ADVISOR. And the placement office at YOUR UNIVERSITY. My institution gives PhD students a booklet of sample cover letters for different academic jobs and CV guidance.
What is the duration of the Social Science postdocs in the US usually for? Do they interview candidates? Does it involve teaching?
Are you joking? How about you look at the JOB ADS (that would be the place to start, before asking questions here). They will tell you how long the job is for, what the search involves, and whether the postdoc is expected to teach. There is no "usually".