My character won't stop jumping!!!

The problem is that the first time you tell it to jump, you just set the velocity to something.

What you are probably looking for is the AddForce function.

https://docs.unity3d.com/ScriptReference/Rigidbody.AddForce.html

Try looking at the example to get a better idea of how this component works!

 {
     
     public int JumpHeight;
     public Rigidbody2D rb;
     void Update()
     {
         jumping();
 
     }
     private void Start()
     {
         rb = GetComponent<Rigidbody2D>(); // or you can just manually set this in the inspector, as it is a public variable.
     }
 
     public void jumping()
     {
 
         if (Input.GetButtonDown("Jump"))
         {
             rb.AddForce((Vector2.up * JumpHeight));
         }
 
     }
     void FixedUpdate()
     {
         jumping();
         
     }
 }
 

This will make it so that every time you press spacebar it jumps - what you were trying to do with the velocity.y = 0 would let you be able to spam the spacebar and jump endlessly, every time the jump apexes. Try instead a character controller, it has a built in grounded function. If you want to go full custom, however, you can use either raycasting or triggers or collision detections. The choice is really yours, if you want more help on any aspect of your character jumping just tell me!