After your result you have to apply for JoSSA counselling. You had to register for JoSAA counselling by entering their JEE Main Roll number, password and security pin. If you had registered for JEE Advanced, then you had to use your JEE Advanced roll number. After the registration, you had to fill in their choices of courses.

To check the complete process you can refer below link

JoSAA Couselling

Good Luck!!