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.
an 'c' is the 2 character in the plain table
you get the 'v' as third characters of the secret table