yoga is more therapeutic... It helps you become more aware of your body's posture, alignment and patterns of movement. It makes the body more flexible and helps you relax.. to feel more fit, to be more energetic, be happier and peaceful... if you want more of a workout kinda thing try pilates.. The emphasis of the exercises in pilates is to strengthen the abdominals, improve posture, stabilize and lengthen the spine, improve balance and overall strength. Pilates gives you a longer, leaner, dancer-like line. My gf does Yoga and I did pilates for a while to lose some weight.. pilates can kill you the first few weeks... >_>

hehe, I don't think you tried hard enough, especially when resorting to here of all places, but good luck nonetheless. 

A quick google search actually found answers to your questions, so yeah you probably could have saved time.

The quick answer is you can begin any variety, you just may not be able to do everything at first. Practice the poses you can do, and try easier versions of the ones you can't.

its either tai chi or yoga? i say do both. tai chi may lossen you up enough for yoga, but i don't know as i haven't dabled in the arts in a long time.

it all depend on wht your trying to get out of it.

To answer your question: you don't need to start with one type to move onto another.  Jump in to anyone that you like.  But I would strongly entourage you to start with a local beginner's class.  Start say once a week, and if you aren't very flexible you might be a bit sore for a few days afterward.  Sometimes you can get them quite inexpensively at a Community College.

You can also supplement with things, like, The Biggest Loser with Kinect for the Xbox 360, and it has some Yoga in it, and even Yourshape on the Xbox/PS3 and My Fitness Coatch for the Wii all have yoga routines. And even DVD's and video workouts.

But do yourself a favor and take some classes in person, so you don't develop bad habits and end up hurting yourself.  Be patient, it took a while to get in the kind of shape you are end now, and it will take some time to work out of it.

It's used to help many people with back and other problems.  It's a great ancient practice.

And it's way to early to be worrying about being 'too thin.' 

Zappykins

PS good luck!