You probably won't.
A friend of mine is good buddies with the bass player from August Burns Red and I've chatted with him several times about getting photo passes and he's always cool about it, but that's one band I've never been able to shoot. I may start working for a local online publication that will give me some sort of credentials, so that'll help me out. The problem is, you have to build a portfolio first.
I suggest if you're in an area where there's multiple venues, check out which shows are playing when and go down the list e-mailing people. I'll usually send out 10-20 e-mails at a time and I e-mail bands I've never heard of or that I don't listen too just for the experience of it. If you just limit yourself to certain bands, especially popular ones that sell out venues, then you're not going to be shooting very many live bands if any. Also, contact local bands and work with them.
But like I said earlier, you'll be limited with your camera as to what you can shoot. I'd recommend something like a used 30D - 60D (or equivalent Nikon) depending on your budget and something like a 20-35mm prime and a 50-85mm prime. That'll be your cheapest route for a capable camera and lenses, IMHO. I use a 17-40 f/4 and 24-70 f/2.8 on a full frame sensor for 80% of my photos and I shoot at larger clubs around DC and Baltimore like Ram's Head Live, the Recher Theatre, Club Sonar, and others and when you're right infront of the stage, you're so close to the action that anything getting past 70mm, especially on a 1.6x APS-C sensor, will feel way too long. /super run-on sentence
I started with a Canon 300D OG Rebel and an 18-55 kit lens with dreams of being some awesome concert photographer and over the 8 or so years since getting serious about photography, I've managed to aquire some good gear, learn from some amazing people, and have some awesome opportunities to photograph people, places, and events I would have otherwise never even seen. It's not a short road so as long as you don't expect overnight results and you are sincere enough to work at it, you'll be able to do some really cool stuff.