Hello Drishti Mohanty,
Please supply a tableau workbook providing sample data showing what you have tried so far and where you are stuck.
It is a holiday weekend for many so there will likely not be a quick response. The more information that can be provided and the addition of sample data will make it easier for the community members to assist.
As Patrick alluded to earlier, without knowing what you are facing, there is little help others can offer.
You may also want to look at Level Of Detail calculations Top 15 LOD Expressions | Tableau Software
These have some good examples on Cohort analysis which translate to the retention analysis you are performing.