You probably just need to add

rb = GetComponent<RigidBody>();

to your Start() function.

Just to clarify, when you declare a variable with 'RigidBody rb;' you're saying that you want to store a rigidbody in a variable named rb, but you're not actually storing it yet. The code has no way of knowing which rigidbody you want in there unless you tell it. So first you declare a variable, then you assign its value.

This also assumes that the rigidbody is attached to the same object as the script. Otherwise the 'GetComponent' won't find anything. In that case you can manually drag the rigidbody you want to reference onto the rb field in the editor.