You choose all targets for Magma Opus as you cast the spell. This includes up to four targets for the first effect and two target permanents for the second effect.

You also choose how the 4 damage will be divided among the first group of targets as you cast Magma Opus. You must have at least 1 damage dealt to each of those targets.

You must choose two different target permanents for the second effect. You may choose permanents that are already tapped.

The same permanent can be chosen as a target for each of the two instances of "target." That is, you can have Magma Opus both deal damage to a creature or planeswalker and then tap it. (We're having a hard time imagining why you would, but you're the brilliant student.)

If somehow all of Magma Opus's targets are illegal as it tries to resolve, none of its effects happen. You will not create an Elemental token or draw two cards.