3D Lighting/Spot Light

Start by resetting your spot light's transforms and move it into place. For rotating you would ideally parent it to a Null to avcoid muddying up your transforms again. You may want to start by reading the online help on some AE basics. None of this is complicated, but of course if you just keep watching tutorials that don't really explain anything (the one you linked is particularly terrible), you'll never understand what's going on.

After Effects User Guide

Mylenium

I'm not sure what you are trying to do. Lux and Shine can produce volumetric lights, but they are paid 3rd party plugins, and you have to buy them. There is no volumetric lighting effects, like the beam from a lighthouse, that come with After Effects. You can fake it pretty well, but it takes some doing. The Spotlight in After Effects will light up 3D layers and cast shadows. 

I'm not sure what you are trying to simulate. If it is the beam from a lighthouse, you need to place the light in the same position as the lighthouse on a 3D layer. You have to build your scene, then rotate the light, and it will light up other 3D layers in the scene. To simulate the beam, you can use a blurred 3d layer parented to the light set not to accept lights. It will take some fiddling.