I need some help with basic things

So i just downloaded Unity and i want to change the color of my cube, plane and skybox

I know this is a very simple question but i have not been helpful finding it.

You need to watch the tutorial videos. Start with the Editor and move on to graphics, watch them all or were needed.

If you want to assign a “color” to a cube, you need to make an appropraite material and assign it to the GameObject. There is a tutorial for that:
Material Tutorial, this also applies to a plane(not the flying plane, but the object type).

Skyboxes are just a bunch of images in a cube. There are TONS of videos to teach this to you: Quick Google Search