1. How do you find the areas of shapes that are not made up of squares or rectangles etc.?

You have to make approximations.

For example in a shape like a star, break it down into a number of rectangles and triangles.

2. What is the area of a rhombus?

There are two ways to find the area of a rhombus:

Multiply the base by the height 
Calculate half the product of the diagonals.

3. Which units are used to measure the area of a country?

Square kilometres (km2)

4. What has to be done if one measurment is in one unit and the other in a different unit?

Before multipying, both measurements must be in the same units.
e.g. If the length of a rectangle is given as 3m and the width as 75cm, one way would be to change 3m into 300cm. then the area can be found by multiplying 300 by 75 to give an area of 22 500 cm