How do I become an interior designer?

Well best advice is to start indoors. Do some stuff for free for friend or neighbours or age concern type groups and take pictures and build a portfolio of you work them apply to design firms and you have something to show them.

Maybe you can do a design course as well but if you do apprenticeship with a design firm they will train you to develop you. So hope this start the ball rolling for you. Good Luck.

You can't call yourself a designer with out going to college for it and passing exams, you can look up the type of classes at UCLA extension, it's a masters actually. Anyone can be a decorator, it's a very tough field to get a career in. You need to know contacts, how to Bill, tax ramifications, you need a business license and a resale license, possibly insurance, just to start

Interior Design is an Engineering specialty. You need to go to school for the degree.
I believe you mean Interior Decorator, although many decorators call themselves 'designers' and get away with it. It's just pretentious. You can get training for that at a trade school or college, as well.

since you have no experience, make a portfolio of your work and shop yourself around to interior design studios, staging businesses etc. You will probably start at the bottom, schlepping furniture for staging but you'll learn a lot and gain some work experience. Good luck.