If you don't memorized all of the song but in progress, your sentence is gramatically correct. But word 외우다 is commonly used than 암기하다. In korea, 암기하다 is normally used when memorizing something written text for test, such as math fomula, korean words, etc. There is an example : "한국 학생들이 영어 단어를 암기하고 있습니다." means "Korean students are memorizing english words." Of course, you can use 외우다 instead of 암기하다, because the word 외우다 is a comprehensive word of 'memorize'. Except of this case, 외우다 is used whan saying 'memorize'. So I recommend to use 외우다 in your sentence, like "나는 2NE1의 "내가 제일 잘 나가"를 외우고 있어 "

I'm not a native speaker, but I would say 외우다 is more common than 암기하다. They have pretty much the same meaning, but 암기하다 is more formal since it contains hanja. I would also phrase it as "나는 2NE1의 '내가 제일 잘나가'를 외우려고/암기하려고 하고 있어.' because '암기하고 있다/외우고 있다' means that you have already memorised something and know it off by heart.