I'm new to extension, I want to convert my object from NAV18 to Dynamics 365
Not sure what your question is. Do you want to migrate a NAV 2018 extension to the newest version of Business Central (aka BC 2019 fall release) or is it a classic C/Side customized solution that you like to "upgrade" to Business Central?
The first is very simple (unless some of the dependencies are "broken" or changed).
The later depends on what your changes are today. If you code is "clean" or you only have new objects, then it can be easy, if not and you have 1000's of changes then it can be a huge project.
Classic C/Side customized solution NAV18 to "upgrade" to Business Central, Dynamics 365?
I have customized tables, pages, reports, codeunit and XMLport.
Ok. May I suggest that you start by watching this video by Daniel Rimmelzwaan:
that is a fantastic video by a wonderful teacher ;)
I will get back, thanks