Hi everyone.
Im new to blender and I really need your help.
Im trying to do a character rig. I finished the first rig, following video step-by-step, and everything worked fine.
And now I’m working on the second rig. After I apply IK to any bone rig acts like crazy. It shakes and spins randomly! I tried everything: new armature, new project, reinstalling blender. Nothing works. Help me please. 
There are two items related to IK constraints:
Target / Bone
The bone is the item you place the constraint on, and target is the first item you select inside the constraint.
You can’t have the Target be a child of the Bone.
The target also can’t be a child of any of the bones that are in the IK’s chain length. You can see this better by looking at the yellow line coming from the IK bone when it’s selected.
A quick test/setup for IK would be this: select one bone, press Shift+I, then select New Empty Object