How much should a fitness trainer charge and do you need any certificates?

I want to become a fitness trainer but I'm a bodybuilder so I focus more on heavy weights low reps. Can you become a fitness trainer just for building muscle only? I don't do cardio at all!!! and do you need some sort of certification to become a fitness trainer becuase I've been bodybuilding for 3 years and know the different muscle groups and the machines. And how much should a fitness trainer charge I hear they get paid well.

Although there is no law requiring certification, you need a national certification in order to get insurance and/or work for a reputable gym.  A national cert is once like ACSM, CSCS, ACE, NSPA.  Generally, a nationally recognized body where you must take a proctored, closed-book exam.  I say that because there are a number of certifications out there that have take home tests or no test.  Generally, they are a crock.  Expect to spend between $400-1000 USD to get a good personal trainer cert.

As far as practice goes, it doesn't matter what your fitness regimine is, as long as you give clients a workout plan that is tailored to their needs.  Generally, trainers charge between $40-100 per hour for their services.  If you work at a gym, you will keep anywhere from 50-80% of what you take in.  A full time trainer can make around $30,000 per year to start, if you are a good salesperson, and a well trained trainer.  As you develop your business, and your reputation and education increases, you can expect to make more.  If you reach a point where you can own your own business/training studio, you can make some decent money.  But my advice is that you pursue personal training as a side job.