Translation:저는 노래를 합니다.
을/를 are object markers. They are used to mark the object on which the verb acts upon.
"저는 노래를 합니다." can be more literally translated as "I do the song." Since "song" is the object of the verb "do" it gets marked with the appropriate variant of the object marker.
Consonants ㄴ and ㅁ that start a word can be "denasalized" to a sound resembling the English /d/ and /b/.
Your tongue and mouth movements don't change dramatically like between the English /b/ and /m/. The only thing that changes is that air doesn't have the time to wind-up and flow through your nose when you pronounce the sounds. You can replicate this by pinching your nose when sounding out the word.
Without it, the sentence would just be "I song", which isn't right in English or Korean grammar. You can say 저는 노래합니다, "I sing", or 저는 노래를 합니다, "I do the song", or even 저는 노래를 노래합니다, "I sing a song". But there needs to be a verb or the sentence doesn't work.
In this sentence, 노래 is behaving as a noun (it has the object marker, 를), so 합니다 is required to complete the sentence.
저 and 내 are pronouns, while 자 and 재 are not. Korean pronouns are quite extensive but here's a quick rundown of the first-person pronouns:
- 저 is
formalpolite "I" or "me"
- 나 is
informalnonpolite "I" or "me"
- 나 + subject marker = 내, can also be used as possessive pronoun (나의)
- 저 + subject marker = 제, can also be used as possessive pronoun (저의)
- 나 + topic marker = 난