I want to develop a software in .NET and it renders 3d models of JT files.
Is there an API I can use for that?
I have heard of the JT C++ Toolkit, but I don't know how to obtain that.
The JT Toolkit is a C++ API. You can contact your Siemens sales representatve to start the process for obtaining the toolkit. If you don't know who that is you can fill out a contact request from the link on this page
If you work for large corporation, there is a chance your company already has access to the JT Toolkit. You can check the JT Open Program member page to see if your company is listed.