rigging new bie help

new to rigging in blender…and a bit lost…

how do i set up an inverse kinematics?

parent lets say a bone to a nurbs circle?

Any tutorial rigging arms and legs will have IK’s http://cgcookie.com/blender/category/tutorials/rigging-tutorials/
For curves and bones. The bone is normally the controller and things get parented to it.
You say " lets say" do you have a project? I hope this is not just random questions.
If you have not searched basic blender tutorials I would do that.