I'm assuming that you're working on the project described here:https://www.sciencebuddies.org/science- ... er#summary
A great way to get started with this project is to study the information provided in the background section, the bibliography and the procedure section. The background section defines the Caesar cipher and describes methods that can be used to decrypt coded text. The procedure section describes tasks that a computer program should perform to encrypt and decrypt text. Of course you have to write the program that accomplishes the tasks outlined in the procedure.
I hope this helps. Please ask again if you have more questions.