This is going to be pretty short and sweet.
Wear anything you're comfortable in.
That's really the most important thing. You probably already have clothes you can start in: sweat pants, stretchy pants, heck - even pajama pants! T-shirt, tank top even thermal top in the colder months are good tops.
You want something you can move in, something not too flowy so it doesn't obstruct movement, not too slick so you can keep your foot on your leg when you want to, breathable and no snaps or tailored waistbands to pinch your middle in twists.
If you're looking, Target (or your favorite discount retailer) actually often has durable, good looking yoga togs. My go-to brand is Prana because they're durable, soft, comfy and responsible with materials. I'm an ambassador and so receive discounts on my personal clothing, but not affiliate rewards. I just really like their stuff. You can get most of it at REI as well. Prana has great mens yoga and workout clothes, too, as well as mats and props.
If you want to check out what I've written elsewhere about yoga clothes, you can check out some of these articles at newbieyoga.com. But really, what you're wearing will do in most cases. How about just one Mountain pose? See, simple as that :-)