Projectile moving in a angular direction

I am trying to fire a projectile in an angled direction with some speed. Currently, i am doing it by using rigidBody.AddForce and internal logic of increasing and decreasing x and y speed.

Below is my code:

 if(speedY >=5 && speedY <= 10 && !limitReached)
             {
                 transform.rigidbody.AddForce(speedX, speedY ,0.0f);
                 transform.Rotate(new Vector3(100,0,0) * Time.deltaTime);
                 speedY += 0.3f;
             }
             else
             {
                 limitReached = true;
                 transform.rigidbody.AddForce(speedX, speedY ,0.0f);
                 transform.Rotate(new Vector3(100,0,0) * Time.deltaTime);
                 speedY -= 0.3f;
             }

I find this very crude and want to use EulerAngles and Quaternion functions to move the projectile in a direction and come down as well. But i am not able to figure out anything rather i am very unclear about them. The documentations doesn't talk much apart from few lines.

How can i specify an angle so that the projectile is fired at that angle. How does it make sense ? For example, i say Quaternion.EulerAngle(0,0,45) then how the angle is created and how does it get applied to the transform ?

It would be great to get some explanation which would clear my concepts.