Hi Suchismita Ghosh,

You can check their "input library" page to catch some initial general ideas about ORCA methods:

https://sites.google.com/site/orcainputlibrary/setting-up-orca

but it is really important to study ORCA manual, which contains the theoretical details of such methods:

https://orcaforum.cec.mpg.de/OrcaManual.pdf

For first steps you can check this tutorial.

Will guide you to some example inputs on how to use the software.

http://www.molphys.org/orca_tutorial/orca_hands_on.html