I shoot 6-8 clubs a week as my main income source, in the nightclub industry (same as all industries, i suppose) its all about networking and getting to know the right people. Just go out there, start talking and meeting promoters, clubs etc and eventually you will find someone that admires your work.
It is long, and hard and involves ridiculous hours (sometimes from 11pm - 6am), but I couldn't imagine a better job, great environment (for some people atleast), free drinks (that comes later!), gorgeous females and just a great all round job.
To start off with I would recommend and this is how a few people I know got into the industry including myself, is do a few contribution shoots. There are many websites out there looking for galleries of nightclubs, in Australia the main website is InTheMix and every week they send out an event list for photographers to contribute to. It's a great way to improve your skills and your portfolio as well as when your out and about you will meet club owners/promoters who will enquire about you and what you do.
If you put in the time and the effort, it will pay off however just don't expect to get paid straight away.
As for equipment, I find that most people that get paid for nightclub photography will have either a 17-55 f2.8 or 24-70 f2.8 with a xxD body. That's just from what I've seen however many shooters out there are simply there with a 400D (aussie model), kit lens and 430ex flash. You will also find out that lighting in nightclubs is just unbelievably frustrating, especially jumping from club to club and getting your settings for each one. Some you can't bounce because of high ceiling, some you can.
Research your flash photography, it will make or break your photos in a nightclub environment
Feel free to PM me & I'll give you more info on payment, techniques n things
Regards,
Michael