@Mark Niman - welcome to the InterSystems Developer Community!  

What do you plan to do specifically with InterSystems technology?  The answer to that question will impact where you should focus.  

My suggestion would be to go to https://learning.intersystems.com/totara/coursecatalog/courses.php, and start by filtering by the technology of interest on the left-hand side - unless you are going to have a specific focus, I would start with "InterSystems IRIS Data Platform".  Then select "Learning Path" and "Interactive Course" from the next set of filters to see guided material for you to learn IRIS. 

Alternatively, if you go to https://gettingstarted.intersystems.com/ and click "Start Coding For Free", you will get set up with an online container and step by step tutorial to start to expose you to parts of full stack development with InterSystems IRIS.  

Hope that helps get you out of the gate :)

@Abhishek Raj - start with reviewing the documentation for new features, and Upgrade Compatibility checklists:

This will be the most comprehensive collection of suggestions, things to know, and 'gotcha's' to be aware of.  

Beyond reading the docs and seeing what is applicable to your environment, do a practice upgrade of your application and TEST, TEST, TEST :)  

Lastly, make sure you review the supported platform list for 2018.1 and 2015.2:

2018.1: https://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY...

2015.2: https://cedocs.intersystems.com/ens20152/csp/docbook/DocBook.UI.Page.cls...

Hope that helps!