How cann I create flat objects to be visible from all sides?
I mean fence, leaf, grass, paper and others?
I think creating a plane with double sided normals in a 3d modeling program should work i think. alternatively you could add a white alpha chanel to your texture and use the shader (particles → alpha blended)this will render the image on both sides.
if you add black to any area of the alpha channel then the engine will render that area of the texture invisible and shades of grey will render the texture transparent like cloured glass the darker the grey the more transparrent the image 
i absolutely love alpha chanels for this it makes image manipulation really easy and allows you to give images shapes without having to model different planes for different shapes like bilboarding, ect which is wat i think you are looking for
If you’re asking how to keep the object oriented towards the camera, then ‘billboarding’ is what you’re looking for. (Search the forum for ‘billboard’ for more info.)