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The answer? Not in these forums! Nah lol, start with some tutorials… sry no links 
Please try to post “correct” questions…at least make them meaning “something”.
If you add a “Plane” to the scene and place it in front of camera’s view, it’ll be “always” there!
If you move the camera, you most parent the plane to the camera, with Ctrl-P keys.
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Use the Halo setting on a plane, and make sure the visible side of the plane faces the negative Y axis.
If you are using this method for bushes or something, set the plane to Bilboard instead of Halo. Halo faces the camera nomatter what, but bilboard only rotates it on the z-axis.
If you are making a HUD, I would suggest an overlay scene.