Students will start by reviewing the motivation for each number type. For example, whole numbers for counting, rational numbers for sharing equally, etc. Then students will see the motivation for the complex numbers. They should also be told about the fundamental theorem of algebra, but not instructed to use it. They'll also learn to identify the real and imaginary parts of complex numbers, and when a complex number is purely real or purely imaginary. A fantastic lesson can be seen by watching this, then this.