What should a young man do if he fondled a girl while naked and ejaculated without penetration, so that Allah may accept his repentance?

What you did, secluding yourself with a non-mahram woman, touching her, listening to her, and exposing your 'awrah to her, is a serious matter. It is incumbent upon you to repent to Allah Almighty with remorse, by ceasing these actions, and by resolving never to return to them. You must cut off all ties with the girl, close every door that leads to temptation, and increase your seeking of forgiveness and performing good deeds. This is in accordance with the Almighty's saying: "And those who, when they commit an indecency or wrong themselves [by transgression], remember Allah and seek forgiveness for their sins – and who can forgive sins except Allah? – and [who] do not persist in what they have done while they know. Those – their reward is forgiveness from their Lord and gardens beneath which rivers flow, wherein they will abide eternally; and excellent is the reward of the [righteous] workers."