The standard order of teshuvah (repentance) for sins between man and God is: confession, regret and resolution never to repeat the act in the future. One who willfully listened as someone related loshon hora is required to engage in this process.
If one actually believed the loshon hora, then the above steps must be preceded by a successful effort at convincing oneself that the information was not accurate. This applies even when the halachah permits listening to the information for a constructive purpose but does not permit believing it as fact.
Generally, sins between man and his fellow require that one ask forgiveness. However, this does not apply when one has accepted loshon hora. So long as the listener has not acted on the basis of the report in any way that was harmful to the subject, there is no need to approach the person for forgiveness. In fact, the person should not be informed that loshon hora was spoken about him, as it would serve no purpose other than to hurt his feelings, and may involve rechilus (speech which causes hatred – See Day 118).