Good suggestions so far. Look into ACE www.acefitness.org
; it's a good starter certification that will provide you with the basics. More respected (and challenging) certifications come from NASM www.nasm.org
and NSCA (can't remember the url...Google National Strength and Conditioning Association.
Also understand that becoming a successful trainer is very difficult. Having an aptitude for fitness is great, but you must also love working with people and have keen business sense. Trainers burn out early because they got into it not knowing that, essentially, they are a small business. It is said that 80% of new trainers quit after the first year.
As Hndsm suggested, go into local gyms and talk with the trainers. Ask them about the difficulties and challenges as well as the benefits.
From personal experience--it is NOT an easy profession. Sure, it's fun working with people, but there's so much more that goes into it than developing great programs. This is not to dissuade you, it's to help you make an informed decision.
Hope this helps.