Hii!
Square kilometre or kilometer square ( K ) is the unit to measure the area of a given surface or ground or field ( usually, in terms of distance ). To calculate it, you need to know the shape of the area. For example, if it is a rectangular surface, multiply it's length with width. That will give you kya area in kilometres square.
For a surface in square shape, just multiply length of  its side with itself. This gives you it's area.

Hope this helps.
Good luck!