you need to learn the language Python to start. Find some
Python tutorial and install the runtime.
This crypto algorithm is very simple called
caesars cipher where every characters is translated into its "secret". This is easiest done over an array in which the plain table gives the index and the secret table give the character for the index.
example:
an 'c' is the 2 character in the plain table
you get the 'v' as third characters of the secret table