Don’t worry, it is understandable, if you don’t know about cryptographic is hard…
For avoiding ruining the fun I will not give the message, but the cipher used is Cesar cipher than consist on enumerating all the letters of the alphabet from 1 to 27 and then define a number than we will sum to the number corresponding to the letter and then after obtaining the number we will put that letter in the message
For example: Hello World encrypted in Cesar cipher with a number 1 will be Ifmmp Xpsme, so for decrypt it we have to rest 1 in this case to the number corresponding to the letter to return to the original message