Calculate Field Values Using Javascript

Can someone explain how I can use javascript (I am assuming this is the best
way to do it) to add two field values together. I.e. field 1 plus field 2
equals field 3 (this is in a php form)

Also how can I set a field to be a value depending on what the user selects,
i.e. they may have eight items to select from and each one has a different
value.

The above javascript will then use that value to add to another field value
to give a me a total value....