From Engineering to Marketing/Sales

Hi, for the past 15 years i have been always focused on software engineering and i had fun and all, and i can make a living (as i'm doing right now) with a standard job. But i would love to be able to have my one things and make my own money. After trying few things i quickly realized i'm missing marketing and sale skill, which i think nowadays can get you way more money than engineering. What projcet, activity would you suggest to improve at this ? i doesn't need to make money (even if it would be nice), but it would be nice to be able to have quick feedback and be able to iterate fast. I still need to work 9-5 at my current job so i cannot go and and sell staff door to door.

I would learn the marketing skills you're interested in and ones you plan to use in the future.