Which core module to learn first is a great question! I doubt there is actually a right or wrong answer!!
I would focus (first) on whatever area is most applicable to your current company. Maybe you work in a sales focused company so looking the sales app would make sense. Or maybe the service modules for case management (etc) is more applicable. I personally would focus on one of these two first. And then begin to look at other areas such as marketing, field service or project service automation after you have understood the standard sales / service functionality.
Your intended job role might also influence your decision. As if you want to customize Dynamics 365 then that might make you want to look at other things. (Such as creating work flows, business rules, customizing forms etc etc.)
There are a number of certifications for Dynamics 365. Your learning might become focused if you decided to study for one or all of those. As that would give you structure. If you have access to DLP (Dynamics Learning Portal) then that is a great place to start.
Your options include ....
MB2-716 (Customization and Configuration) .... ideal for developers or people who want to customize Dynamics.
MB-715 (Online Deployment) .... ideal for those who need to deploy Dynamics or integrate it with other applications such as email, SharePoint etc.
MB2-718 (Customer Service) - Ideal to understand all things related to the customer service related modules.
MB2-717 (Sales) - The sales app of Dynamics 365.
Personally, from what you have said. I would suggest starting with MB2-717. Then doing MB2-718.
Unless you are a developer / customizer then MB2-716 might be your ideal exam!
Some people have suggested looking at crmbook.powerobjects.com (I work for PowerObjects! and I agree it contains some great content.)
Additionally, if you decide to look at the certification options you will find content on my blog that might help. https://neilparkhurst.com/