You'll need to take a certification course - there are many good ones out there but the top three are probably the ACE, ISSA and NSCA. They are all done by correspondence but you have to take an exam and do a practical in some cases. Cost ranges from $300 to $600. They will get you a job being a PT anywhere.
My biggest piece of advice is to EDUCATE YOURSELF. Don't go into being a personal trainer to get paid to tell people how to work out because you think you know. It is a constant learning experience and you should always be reading up on the newest news, techniques, articles and books from the experts in the industry. Start reading everything you can get your hands on - any information, whether right or wrong is still valuable because it builds your knowledge. There are too many personal trainers today who are useless when it comes to knowing why they do things to their clients.