I have never done it before and I am immensely nervous.

Oh no! Don't be. BUT...if it's really bad, arrive early, do not sit in front, sit near the door if you need to make a swift and undercover exit (panic attack, withdrawal, et cetera). I think you'll be fine, but it's a tip that has helped me to be near the exit and out of the front when I thought I might have a 'hiccup' in the middle of a class.

I am trying yoga because my doctor said it would help my scar tissue in my back and help with my anxiety, is this true?

It is possible, but bodies are a case-by-case thing. It's a great idea to try as many tools as you can until you find whatever tool works for your body best.

Yoga for scar tissue in itself...it may depend on when you got injured and consequenty how thickened the tissue has become. I don't know, obviously your doctor does, so don't listen to me on that!

Will Yoga help me cleanse my body of those icky toxins?

I'm no scientist, but I can say psychologically that it gave me a replacement, a way to feel relief that I was previously using substances like cigarettes for. So who knows the science, I'll leave that to the professors, but you can surely be 'cleansed' of the craving to ingest some subtances once you start getting the same relief from yoga which does not have withdrawals like cigarettes.

what else I could wear so that I may feel comfortable attempting this in front of others

If you are self-conscious about your feet, maybe wear skin-matching socks or something along those lines that look like you're barefoot but don't expose you too much. If you are fine with your feet, good for you!

Wear something slightly tight under your shirt or as your shirt. Many women who begin find out the hard way that going into downward-dog (which is the staple of restorative classes) is very embarassing if the woman is only wearing their bra or something revealing under their loose shirt.