There are different ways to get your credits and credibility in the personal training field. If you have a fitness club in mind you would want to work at, then you can contact them and ask what they require for their trainers.
Investigate and choose a place that is accredited and nationally acclaimed, so you can get a good amount of training. Many personal trainers boost their resumes by getting specific trainings, such as working with people with certain diseases, working with the elderly, and certain exercises.
After you have found a way to get your certifications, and you have passed, get a job at a local fitness club. Personal training courses are easy to find and to graduate from, if you are dedicated to the profession.
The rest is basically all about the business, and learning how to succeed. If you can, after a few years while working at a club, think about getting your own club or business. This can increase your revenue, and allow you to expand on your own. You can hire other personal trainers, and work to increase your clientele list. Work on becoming popular with your clients, being friendly, and becoming irreplaceable. If they enjoy working with you, then you will more likely to hold on to clients longer.