2d fire from player towards mouse

I am trying to make a bullet fly in the direction the mouse is located in relation to the player character.

What I have so far:

public class BulletMove : MonoBehaviour
{
	public Rigidbody2D bullet = null;
	private float speed = 5f;

	void Update()
	{
		if (Input.GetButton ("Fire1"))
		{
			Rigidbody2D shot = Instantiate(bullet, 
			                               transform.position, 
			                               transform.rotation) as Rigidbody2D;

			shot.velocity = new Vector2(-1, 0);
		}
	}
}

This shot.velocity = new Vector2(-1, 0); makes me able to shoot to the left only as its hardcoded numbers but how do I get the position of the mouse to act as direction to shoot?

Rather then doing this with the velocity, do it by stetting the rotation of the bullet in your Instantiate.

Quaternion has a function to calculate this for you alreadybuilt in: