Question

how does a mutation happen?

Answer 1

a mutation is simply the substitution of a nitrogenous base at a specific location for another - for example, a thymine for an adenine. It can occur in different ways, the most common is during DNA replication (because of it's frequency), another is during DNA repair after exposure to mutagens or other damage.

There are also various outcomes to the mutation; it can be "silent", "non-sense", etc.

Here is a good resource with lots of reliable information
http://ghr.nlm.nih.gov/handbook