I'm not a very good 3D modeler but can come up with stuff that's not too bad. Often, I'll use a reference image, a photograph, a drawing, whatever I need. Depending on the type of model the method will change. Often I'll choose a couple of views and trace an outline by placing vertices at key locations and link them together. I'll extrude or spin the edges how I need, then do some final shaping vert by vert for as long as my patience lasts.
Mirroring comes in real handy.
Quote: "I lack the formal art school training so doing realistic renderings is simply out for me..."
Schools are good for technique and in the 3D world for learning to use modeling programs well. But ability, creativity, generally don't come from a school. Something that may help is to get hold of some modeling clay. Play with it, shape your imagined 3D objects with it. Look at it. See how the light hits it. See where shadows are and aren't. Look at the different angles.
If you become very visually aware of the physical 3D clay object and fairly dexterous at manipulating the clay (this doesn't mean you have to be good at sculpting), it may help you gain insight on what you want to see in your digital 3D world and improve your renderings.
Enjoy your day.