There's no reason why she can't start wearing a collar right away. Not sure if I'd be inclined to leave it on all the time, but I guess there's no particular reason why not.
What would be useful is putting the collar on her for short periods and letting her drag her leash around for a bit - to get her used to that before you start leash training her.
As for what type - something lightweight and inexpensive. She's going to grow out of whatever you get her fairly fast, so there's no point in it being expensive. A lightweight nylon or leather flat buckle collar is ideal.