Ask HN: Guide to building an online business as a side-job?

I'm interested in creating an online business, as a side-job as I'm working full-time and it pays well.

I'd like to learn how to go from 0 to paying customers. I have no experience in building businesses or selling, but I'm good at web development, design and shipping products.

How can I start? How can I make sure my ideas don't land me without a good pay? How can I grow from 0 to something in the next 6 months, if possible?