Shape is what something looks like. Form is what something is.
Technically Garfield the cat is abstract art. If your cat looks exactly like Garfield seek medical attention immediately for both you and your cat! There are enough cat like features and impressions so that we can relate Garfield to being a cat. His shape is round because he's fat. Round is a shape. Cat is what Garfield is. Therefore cat would be Garfield's form.
There are many types of shapes but there are two basic categories... Organic or Geometric. Organic shapes occur in nature, and are often grown or shaped by naturally occurring events.
Geometric shapes which can be called inorganic or mechanical are often crafted and reflect intelligent design. Most geometric shapes require manufacturing of some sort.
Robots such as WALL-E are built and crafted by geometric shapes. WALL-E if you look closely is a combination of inorganic or mechanical shapes.
Here is an example of organic shape the plant being planted by an mechanical shape WALL-E.
The way you make a complex form is to take basic shapes...
and combine them using lines.
Ever wonder where stop signs came from? Look at the shape of the hand.
The outline of an object is called a contour. It's the line brings the organic and inorganic shapes together.
Just by looking at a contour you tell what something is. What does this contour below look like? If you guessed a city skyline you'd be correct.
You can almost think of how inorganic and organic shapes are added as an mathematical equation. The equal sign being the contour or outline that brings everything together.
Number one rule: DON'T BE BORING!!!
If you are bored so is your audience. Don't copy others, but learn and be inspired by their work so you too can make something that is interesting or cool to you.
Even the most simple task could look awesome if you combine it with something great. Vanilla ice cream can be boring, but when you add candy, fudge, and caramel you got something special.