A common way to get started is on a site like Upwork by bidding on smaller jobs. Do good work, guard your reputation and you will soon have people sending you requests. You will also stand out in a sea of BS proposals when you apply for contracts.

If you approach it like you approached getting an airline job you will quickly be in the top 1% of the freelancer/applicant pool.

Javascript is probably your best entry point in terms of widespread use and pay. Most pilots could learn JS and at least one popular JS framework by June just by trading Netflix for a cheap Udemy course or two...at least to a level to get started.

PHP is another good place to start.