One can join a graduation course after completing 12th. To provide your professional services in Interior Industry, you really need to do a course from a reputed place. Always make sure that you learn more about practical. Many people have wrong perceptions that interior means coloring walls, selecting fabrics etc. Which is wrong. But in real, you need to know about different types of materials, its properties, the way of use it, the cost factor, the execution technology, the project management, electrical/plumbing etc.
Unfortunately, most of the institutes will teach you theories only. Which is not sufficient. Many institutes are offering 6 months certification too. Which is tottally no use. To gain perfection in this line, you need to study for minimum 3 to 5 years and need to gain training for at least 2 more years. You can find some knowledge from Practical Online Interior Training