3AM. Perfect time to write this. Gonna make you work to decipher my dissevered rant, har.
Your first port of call will be to do everything you possibly can in order to build your portfolio. You need strong tracks/CDs behind you in order to promote yourself. Whether that involves splitting studio time fees with bands in order to entice them to track with you, doing some jobs for free, or whatever else. Your best bet is to find the most talented local band you can and do what you can to build a CD for them. The word of mouth which ensues from that will be like the stone falling in the pond, from which all ripples ensue.
It's a very slow, gradual process but you will eventually start receiving artists seeking you out based on whatever aspects of your reputation they've heard about. Whether it's that you did a cool CD for band 'x' or whether you're the cheapest guy in town... you need to make sure they have a bone to chew on.
You will eventually get a to a point... and this point is critical, as it may shape your career path from that moment forth. That fork in the road will effectively ask you what you want to be renowned for. Will it be as the guy who works really fast, and turns over projects on a dime? Will it be as the guy who delivers quality, with expense not being a factor? Will it be as a guy who specializes in an extremely narrow subgenre of music and monopolizes it? Will it be as a guy who is a balanced, all-rounder?
Try to think ahead and anticipate what part of the industry you want to be working in down the track. It's very easy to pidgeonhole yourself as the 'day demo' guy, who gets shit bands to record 6 days a week, yet still barely makes enough to pay the bills. It's also easy to pidgeonhole yourself into a particular genre or aesthetic, and get utterly trapped there. My suggestion would be.... when you hit that moment of clarity and you realize 'hey, I'm actually quite good at this', enforce your rates. Make them sustainable, and don't be afraid to turn away a few dozen artists in order to ensure your long-term success. You will likely go through large periods of drought with this approach, but in the end it's where you want to be. You want your skillset to demand an appropriate salary, and not settle for anything less. It's one of the only ways to stop yourself getting trapped in that grind of working with countless nobody-artists who demand too much for too little. You will become jaded, and very likely will leave the industry regretful. Think about creating a lasting career for yourself. Find your niche, work out what you're good at, and capitalize on it.
Word of mouth is still the #1 form of advertising in this industry. Leave good impressions with your clients. Make sure they're very happy with what you've done together. The internet is a very strong auxiliary, as it opens up the entire world to you. If you don't mind specializing as a mix or mastering engineer to a degree, then you can cultivate a great many clients from abroad just advertising on forums much like this one.
Be ready to make significant sacrifice in your social life. No girl you ever meet will be happy with your job, nor the hours you keep. Find a way to manage that, or get used to being single. When everyone is out getting shitfaced on a Friday night you'll be pushing through final revisions on your last mix, trying to make it all count before your mastering deadline on the weekend. While everyone is coming home at 6pm, you'll be saying 'see you tomorrow' to your tracking clients, then follow that up by booting up a mix you have going concurrently, and finishing up in the early morning, only to start tracking again 6 hours later.
It's a real labor of love, so if you want a lasting, professional career, you need to make sure you want it more than just about anything else out there, because you won't be able to have it all. At least not for the first decade or two.