When I started yoga, my instructor made it clear that I needed to wear clothes that allowed her to see my form, so she could correct alignment as necessary. Since then, I've realized that I really like yoga tops for yoga, because they are long enough not to show skin in back no matter what position you're in. But if you're going for the first few times, put on some gym shorts and a babydoll T and ask the instructor to correct you, and if they can see well enough to be able to do that. Probably my favorite yoga teacher wore plain white crew neck T-shirts/undershirts. When he moved, we all wore them for his going-away party.
