Welcome aboard
Well, basicly look good at highlights and shades on photo's try to eliminate what are shades from other objects.
Then approach such highlights and shades.
Examples for your images: Air intake of the F-5 looks a bit flat due to lacking highlight at the top of it.
Maybe a shade of the wing reflecting on the fuselage adds some effect to the Saab.
Asking for general tips often does not work very well. Show work ask for specific tricks and people tend to answer much more.
