How do I become an interior designer?

This is seemingly a simple question which has a very complex and deep answer that has no quick solution. The best way to start out in this field is immersion. Read and watch EVERYTHING you can!! Your mind needs to learn ideas, concepts and teachings that are commonly used in interior design. If you have no previous working experience find a design firm that is filling positions for interns or assistants. Ask tons of questions and be very observant. There is also tons on information on the internet. If you want it bad enough you will find all different avenues of learning.

To John's sage advice I would add, consider attending (in-person or online) an Interior Design School and see how you react to the challenges and tasks presented to you. See if you truly have the passion!

Honestly, I believe that we are born with certain skills. My skill has always been creativity which after years of having different work experiences I decided that interior design was my forte'. It started in my childhood, I was aware of beautiful surroundings and always had good taste. For me personally, it was working with clients one at a time that gave me the skill and confidence to walk into a strangers home and figure out what they needed to do to make there particular space comfortable and enjoyable for them. My first client had no idea that they were my first!!  From that it lead to many, many more. Confidence is key as is knowing what products are available depending on the budget.