Retrieve content type column name in a WebPart

Hello, 

Here is my first experience in Sharepoint Online...

I first created a content type for billings. Then created a document library and finally created the association with the content type. So far so good. 

Now i have to develop a form in a webpart. The form is generated based on the column names of the content type.

I have several issues : 

1.  How can i retrieve the column names from the content type used in a specific library
2. How can i manage a form in webpart ? I tried to paste a form and unfortunatelly the tag is removed. Rather annoying because i'm using JS to retrieve dynamically formElements. Anyway, if the form is generated dynamically, i should manage.

I'm rather stuck here

Thanks