How to build a shipping plugin on Shopify using NodeJS?

[prathamesh86A]

Hello Everyone ,I am building a shipping platform which works as a plugin in Shopify. Where the sellers can integrate our service in their Shopify store. 

For that we need to fetch the order details from the sellers store,

I am doing this by integrating our app with Shopify using NodeJS. 

Can any one help us to understand the flow from start to end to create a plugin. I am stuck on the authentication page as page is not redirecting to our platform. 
I also have some other queries which are listed as follows :
1. want to know how access token is generated is it based on the scopes we want to use ?
2. for a particular Seller how many access token we can generate ?
3. want to pass the details of the user after successful installation to our service portal how can we achieve that ?

any reference to the documentation or any help will be appreciated .
Thanks in Advance.