Shame on you Velusamy :( anyone using a GCMS for identification should know how to do that. Shame again for not using Google.

https://en.wikipedia.org/wiki/Kovats_retention_index

(and many other websites)

Of course to be able to calculate Retention Indices you should have run n-alkane standards when you analyzed your samples. If you haven't done that you can't calculated Retention Indices.

BTW if you are using a GC temperature program it isn't called a Kovats Retention Index but a "van den Dool and Kratz Index". Kovats is only used for an isothermal GC temperature. You use different formulas for the Kovats RI and "Dool and Kratz" RI.

Regards, Alex