There are two main types of ankle holsters: the ones that ride down on the top of your shoe, and the ones that are held up by a garter around the top of your calf. I prefer that kind. Some manufacturers have an optional strap.
If you're old enough for your legs to swell by the end of the day, a Velcro/elastic strap set is much more convenient than a fixed arrangement.
Ankle holsters are sold in left and right hand versions. Which is which depends on the manufacturer, and whether you intend to cross-draw or draw from the same side. I have found through experience that if you're cross-drawing, the butt of the pistol should point behind you. If you're drawing from the same side, it should point to the front. You might want to experiment a bit with an imaginary gun to see which is more convenient for you.
You *can* futz around with your pant leg and dig the gun out, but for practical purposes it takes two hands to draw from an ankle holster - one to pull your pant leg up, one to grab the gun.
I carry an LCP in a nylon ankle rig. I've seen ankle holsters that are supposed to hold a 1911 or Glock 17, but I don't think I'd want to carry anything larger than a J-frame or a compact/officers style auto that way.