encryption/decryption help

I have very recently learned c# and was wondering on how to implement a way to do encryption/decryption. what im trying to do is use the Blowfish-compat algorithm with the key "mothking". this is for a program where the user inputs a txt file and it either decrypts or encrypts it (depending on the user's choice) with the Blowfish-compat algorithm and the mothking key.