I'm assuming you're referring to lyrics, and not the actual playing of music.
In that case, repetition. Sing along with the lyric video. Group the lyrics by section. After a few times, focus on the lyrics you have more difficulty remembering, and sing those verses more often.
Sing the next line before the singer says it. This way, you anticipate what the words are, and can correct yourself if needed. Once you got this down while listening to the music, sing acapella. If needed,learn those specific verses, repeat the process from there