Hello!I’m new in Unity and i would like to know how can i make my planes to be double-sided 
As far as I know, you need to duplicate your polygons and reverse the order of their vertices. I believe this is due to the nature of hardware rendering.
Thank you!
Not quite - with the rendering state set up appropriately, rendered triangles can be double-sided.
It should be possible to find or customize a shader that will do this. However, duplicating the geometry may be easier (and may also be preferable in some cases, such as if lighting is involved).
From what I understand, backface culling would cause the polygons to be ignored by the renderer before it even got to the shader to render. Correct me if I’m wrong though.
No, Jesse is correct. It is possible to disable backface culling in a shader. As he already pointed out, this has several disadvantages (having to write/find the proper shader and incorrect lighting if lighting applies), whereas your original suggestion to duplicate triangles has none. Even in terms of performance, there is no significant difference, so it is usually best to go with your suggestion.
No, Jesse is correct. It is possible to disable backface culling in a shader. As he already pointed out, this has several disadvantages (having to write/find the proper shader and incorrect lighting if lighting applies), whereas your original suggestion to duplicate triangles has none (i.e. unless you count the modeler having to some extra work, but I normally don’t :P). Even in terms of performance, there is no significant difference, so it is usually best to go with your suggestion.
Good stuff tomvds; you are a ridiculous source of information.