how and when we can fill the form for internship

That depends upon what kind of internship you want to do. If you are looking for an intership that is work from home and your college doesn't sponsor intership process, you can go for INTERSHALA app which provides many interships which are paid and give you the certificates and letters of recommendations. Hence, if your college does not provide internship, go for this app and find your desired internship. It is a great app with genuine companies looking for recruits