Perez,
Wow, and thanks for bringing up yoga!
I don't have any simple answers, any more than any of you could answer "What's tattooing?" in a few sentences. I am a yoga teacher, and have been studying for about 15 years or so, and I've tried many styles and gazillions of teachers. I'm going to be honest, there are a lot of crappy teachers out there, and if you suspect that a certain teacher isn't for you, than they're probably not. One huge turn off for me is if someone is speaking in a sing-songy "yoga voice," instead of their own authentic voice. You might need to try a few different studios and teachers until you find the right fit, or maybe you'll get lucky the very first time. But ask around with everyone you know who does yoga, and hopefully someone will at least point you in the right direction. Try to find a place where the students and teachers are talking and laughing instead of trying to look "spiritual."
Mario, I did make a Chicago teacher recommendation to Nick, maybe you guys could do some semi-privates with her? You also might want to check out some Forrest yoga or find some basic vinyasa flow classes. And as far as "new-age" bullshit, there really shouldn't be any. Yoga's a super old system, and I know the concepts of chakras and karma have been misused by some new age folk, but chakra discussion first appeared in the Upanishads, and pre-dates Buddhism, so it's far from new. And I'm going to let the cat out of the bag, the translation of the sanskrit word "yoga" is "to yoke," meaning to yoke oneself/to unite with the divine. But don't worry, it'll take a lot more than a few hours on the mat to get there, there's no threat of instant enlightenment. In the mean time, you find relief from your body's aches, have more energy, get better sleep, be stronger and have more endurance.
Personally, when I'm teaching, I'm concerned with my students breathing and physical alignment, not if they aspire to align themselves with the divine. That's their personal business. A good teacher will meet you where you are, and wouldn't try to impose any of their personal beliefs on you. If one rubs you the wrong way, move on quickly, it's hard to throw a rock and not hit a yoga teacher these days.
One of my favorite teachers is my friend Stephanie, this is her DVD. It's amazing, and I have a TON of yoga DVD's, and sometimes I watch them as a sleep aid, generally not to practice. There is absolutely NO spiritual content whatsoever on this DVD, it is what it's called, yoga for strength and toning, although her core sequence just might have you crying out for Jesus!
http://www.amazon.com/gp/product/B002JYPV7W?ie=UTF8&tag=lasspatat-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B002JYPV7W