So I have this shape that I want to turn into a sign.I want to make a black frame around this on these selected edges using materials.How can I make the edges look like an actual frame in object mode?
If you want to add the frame only using materials, then you'll need to apply an image texture to the material. For that you'll probably want to unwrap your model first in order to be able to better place the image and match it to the shape of the object.
Having said that, it might be easier/better to model the frame with some faces. for example, you could dissolve the four faces you have and then inset the one face you'll end up with after dissolving. Then you can select the outer faces for the frame and assign a different material to those faces.