The simple answer, Esther, is this - don't use it.
You NEVER need to use the word 'upon'. Apart from a few antiquated set phrases such as 'Once upon a time' and 'Upon my word' which you have no reason to use, you can always use 'on' instead.
If you see or hear the word 'upon', just think of it as meaning the same as 'on'. Otherwise, just forget about it. You'll never need to use it.