i'd start out by getting an internship or job at a design studio (or any place you can use your skills like a record label, pr firm, music festival, booking agency, publisher, magazine, print shop ect ect ect and make connections and learn different aspects of the industry inside and out before starting to freelance full time. some good connections and inside knowledge can save you from a ton of headaches.
It all kind of depends on what you're trying to do. If you're trying to make posters and flyers, just start out small. Ask local bands if you can help promote their shows with art, then upload the designs here. If they're good, you'll keep climbing the ladder, and eventually, people will notice your work (this seriously happens all of the time. Tyler Stout, arguably the most popular artist making screenprinted posters today, was discovered via his local, black-and-white flyers here). There are a ton of ways to integrate yourself into the art community online, but the key is your work has to be good. I, and everyone else that helps to promote art on the internet, receive a boatload of emails everyday from folks wanting me to write up their embarrassingly bad prints (sad, but it's totally true), and I don't give them a second look. On the other hand, you can be a complete nobody, and if your work is great, you'll suddenly find people wanting to promote you and offer you jobs. So the first step is honing/mastering your craft. The rest follows naturally.
PS, it's nice to be posting at GP again. That Jazz Junkies thread sucked me back in.