Not going to lie: Most of my clients come from my network and their referrals. Most of my freelancer friends’ clients come from their network and their network referring clients to them. While it is possible to find clients organically through advertising, it’s much easier if you can build a network to pull from.
You can start by simply contacting people you know and asking if their companies need any help. Let them know you’re available. Then, when they ask, be available and solve their problems ASAP.
It’s actually quite a challenge to hold down a full-time job and to freelance at the same time. You might be lucky enough to find clients who don’t care much about how long it takes and are fine to communicate asynchronously through e-mail at your convenience. However, most clients will want to get on video calls throughout the day and will expect the work to be done quickly. Realistically, it will start competing with your day job at some point. You should decide now how you’re going to handle that.
Whatever you do, don’t use your work computer for any freelancing tasks. Neither party may ever find out, but if you get into a situation where it matters then it’s terrible to have intermingled the two.
Finally: Don’t underestimate how good a stable, big company job is right now. Freelancing isn’t really the easy money that some people make it out to be, especially if you’re not actually free during the day because you have a job. If you’re looking for something different, consider just getting a different job.