Where does the comma go when you use the word because?

It could go anywhere or nowhere.

Examples:

'He fell over because he tripped on a loose rug.'

'He fell over, because he tripped on a loose rug.'

'He fell over because, in spite of walking carefully, he tripped on a loose rug.'

'He fell over, because in spite of walking carefully, he tripped on a loose rug.'

'He fell over, because, in spite of walking carefully, he tripped on a loose rug.'

'I love you because you are kind.'

'I love you, because you are kind.'

'I love you because, contrary to what everyone says about you, you are kind.'

'I love you, because contrary to what everyone says about you, you are kind.'

'I love you, because, contrary to what everyone says about you, you are kind.'

All the above would be correct if used in the appropriate context. They have slightly different shades of meaning.

Generally, a comma represents the shortest spoken pause. If you say your sentence out loud and notice where you naturally pause briefly, that's probably where you should put your commas.