I'm not a native Korean speaker (I'm sorry ㅠㅠ) but when my Korean friends ask me I say: 아니요, 남자친구 없어요. (Ah-ni-yo, nam-ja chin-gu ob-seo-yo) or 아니, 남자친구 없어. (Ah-ni, nam-ja chin-gu ob-seo) One is in 반말 (informal; the bottom one) while the other is in 존댓말 (formal).

Using the informal and casual speech, it would be: 아니, 난 남자친구(가) 없어. (아니: No, 난: I, 남자친구: boyfriend, 없어: do not have) "Ah'nee ^^ nahn ^ nam'ja chin'gu up'seo" (^ indicates a short pause) Or "Ah'nee ^^ nahn ^ nam'ja chin'gu'gah up'seo".