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Having the right gear is very important in low light situations. I use a mix of crop sensor and full frame dslr bodies with 1.8f glass, and a 70-200 f2.8 for festivals and larger stages.
Preparation is very important untill you get the hang of shooting live gigs. Do research, see what the band itself posts.
To get into a venue, try joining a local webzine and work yourself up the ladder. Social media is full of name.photography accounts, so try avoiding that. Buy a cheap domain and get a website. Do promotion for the gigs you want to attend and talk to the organisers. Be prepared to do small shows for the first couple of years. (Ive done more than 150 shows for locals before being hired). Small local venues will allow you to take photos during the whole show, while bigger venues might ask you to photograph the first two or three songs only before toasing you and your equipment out of the arena. Happened to me on the Toto show a couple of years back.
Also, the most important part of being hired photographer is networking. Look your best, dont get shitfaced, and try to publish the photos asap. Even if they suck, you will be the first and get the most exposure.