If both the numerator and denominator have a common factor (i.e., they can both be divided by the same number), you can reduce the fraction by that number. It's fastest to use the greatest common factor of the numerator and denominator.
If you don’t happen to know the GCF, it’s perfectly fine to reduce a fraction over and over until it can't be reduced any further.
For example, let's reduce:
reduce by
reduce by
reduce by
In the example above, if you had spotted right away that the GCF of the numerator and denominator is 12, you could have solved this in one step.
Fractions in answers almost always should be reduced or simplified.
Reduce: 

Reduce:  The GCF of and is , so each can be reduced by . 
Reduce
Reduce
Reduce