The best organizations in the country to go through are the ACSM, NSCA. Other good ones are ACE and the NASM. Colleen already posted the links to ACSM and ACE.
Most respectable certiifications will require you to be CPR certified before you take the certification test to become a PT.
The ACSM is the gold standard of fitness organizations in the country. Last time I checked, they require you to have a bachelor's degree in a health related field to take their personal trainer exam so I don't know this is a good option for you at this point (maybe something to shoot for though). The only problem with ACSM is they tend to be a little more clinical (like ideal for working in a rehab setting) than needed and I think the NSCA's certification is a little better for most settings a personal trainer would work in.
The NSCA is also great and doesn't require you to have a degree, but does require a CPR certification. The test is supposed to be pretty hard but this organization is top notch. This is the organization most professional strength coaches are certified through. You can order study materials that are very education through their website.
ACE only requires you to have a CPR certification to take their test. The test takes about 2-3 hours and isn't easy but not as hard as the NSCA or ACSM exams going by what I hear from people who have taken all of them. The only problem I had with the ACE exam when I took it was that it seemed to focus far more on psychological and legal aspects than on anatomy, physiology, nutrition, or biomechanics. If I didn't pass it I would have been bitter.

Their study materialis, like the NSCA, are really good though not quite as current as the NSCA's are.
That's all I can help with because I don't know much about the NASM exam except that they have a home training course that is supposed to be pretty good.
Hope this helps.