formatting - What is the formal way to write a reference in English?

I am writing my thesis and I need to know the correct way of putting the reference.

For example:

This is a sentence taken from someone [2].

This is a sentence taken from someone[2].

This is a sentence taken from someone. [2]

This is a sentence taken from someone.[2]

As you see in the four sentences above, there are differences in the space placement and the dot placement; which one of these is correct? (or are they all wrong?)