How to use a line to aim in Unity3D?

I have this GameObject upon which I click and addForce for it to move, as if I was flicking it. For now it only moves forward. I want to visually aid the player by introducing a line which allows him to aim, by rotating it with mouse movement. The line would be restricted to the plane parallel to the ground and upon clicking, I would use its direction to AddForce in the direction of the line. I've just recently started using Unity and I'm still getting use to things. How would you recommend doing it? Is there any good tutorial about it?

Thank you.