How should i go about encoding string

I'm making a decoding/encoding website in which the key is some weather data from the city you choose.Do people usually use someone else's encoding engine or create their own? And how should i go about it if I decide to do it by myself?

because i know that i can give letters numbers and then multiply/divide by the key to encode and reverse it to decode.But i don't know to seperate different letter values from the whole string of numbers after they've been encoded.

I want to encode and decode data by using a key gathered from an external site based on the user input.