How do I give a realistic appearance to my 3D object's surface? Like, custom textures?

Play around with the rendering style in the Display tab. You can achieve a metallic or brushed finish along with a glossy/matte appearance. These basic settings allow you to tweak how the environment lighting bounces off the objects.
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You can also import an image of the material's surface (like https://www.google.com/search?q=aluminum+texture) into the texture settings of the 'Color -> more colors' menu. Try to get as large/high resolution image as possible, so it easily wraps around the solid rather than giving a stitched mosaic appearance on one face. 

To the insert the image, click 'add' to browse for it first and then drop-down 'no texture' to select 'diffuse'. Click OK to save and the selected object will display its new wrap.
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