Fill contract field on Case creation

Hi,

I'm trying to understand how to create a custom workflow activity/plugin (still don't get which one is better), but i need help with the code, what i need this to do is after a case is created, it will check if the account has a contract and fill the contract field on the case, if not, it would check if the account has a parent account and if that parent account has a contract, use that contract on the case and so on until the account don't have any parent.

My CRM version is 2015 (7.0) on prem.