Hello,
Diploma in Interior Design is a 1 year diploma course. After completion of 19th, you can go for the Diploma in Interior Design but your job opportunities will be very less. Also you won't be able to go for Bachelor's degree or Master's degree after that as they require a minimum qualification of class 12.
So if you want to go for Bachelor's degree in Interior Design you must first complete your class 12th and then you can go for it. After completion of three ears course in Bachelor's in Interior Design, you will get vast job opportunities and also you can go for a Master's degree.

Hope it helps