If all the cards in your library are lands, this card deals zero damage.

As Kaboom! resolves, if you have chosen multiple targets, those targets will be handled one at a time in turn order. Start with targets that are either the player whose turn it is (probably you) or permanents that player controls. You choose the relative order of targets within that group. For whichever target you select first, exile the top card of your library, then Soulfire Eruption deals damage to that target. Repeat this process for each subsequent target in the group. After that group is done, move on to targets that are either the next player in turn order or permanents they control. Again, you choose the relative order of targets in that group. Repeat the whole process for each other player.