what subjects should i take in IGCSE if i want to be an interior designer?

Interior Design does not require any particular GCSEs, but subjects such as Art, Graphics, and Textiles would be appropriate.

Art, economics, business, marketing, accounting are all necessary to run a successful interior design business. Yes, the CAD and Revit software is also very helpful as a way to get in the door at a larger design firm.
Look up Interior Designers in your area and see if you can shadow them for a week or two during the summer, this will really give you a snapshot of the business and help you decide if it is the right path for you.

You need to get light and color in fine art to really understand the basic of this major from there you can go further for space planning and technical drawing classes.

I would suggest taking classes in the following areas:
Basic Drafting, Color for Interiors or Color Theory, Perspective Drawing, Environmental Design, Historical Styles, etc. in addition to the basic core classes, math, science, writing, etc.

Much Success!!

To be a good designer a strong background in art (history,color theory, proportion and scale) and architecture is essential (not necessarily hard-core engineering, but understanding how houses are built is extremely helpful).
A firm understanding of fabric (types, qualities, textures, drape, stability) is crucial; understanding how various fabrics are manufactured is extremely useful in determining why one fabric will suit a project better than another.
Learn how to properly measure and how to calculate a pattern-repeat; don't rely on others to do this for you. Too often a job will go wrong over bad measurements.

Learn how the craftspeople behind the scene do what they do (i.e the upholsterers, the drapers {curtain-makers}, the painters and wall-paperers) try your hand at what they do, seek out their opinions and expertise. You won't know if a job is done well if you don't know how it should be done.
A class or two in psychology is practical, as designing other people's homes is as much about dealing with their emotions as it is with their physical space.
Hope this was somewhat helpful.

A well rounded education in visual arts for example, drawing, color theory, the elements of design, textile science and furniture period research. CAD courses in Revit, AutoCAD, and drafting AR a must. Construction science is another area of knowledge that is helpful. Business and communication courses are very helpful if you plan to go into business for yourself; which most designers tend to do. You will need to know how to communicate and present your ideas via illustrations, written proposals, budget breakdowns and speaking to your client and your project team. Spanish or another foreign language would be a great asset as well. I hope this helps! Good luck!

You should take the Art & Design subject. This subject will help you develop technical skills in dimensional drawing, problem solving, and gain some culture history about art and design.

Good Luck!