'I love you' is not a phrase that most Chinese use in their daily life. It's a phrase used when you feel in love with some one and think about marriage. So, think and think, before use it :)

I love you ==  我爱你 == wǒ ài nǐ

I is 我 in Chinese. It is pronounced as wǒ, the third tone.
Love is 爱. It is pronounced as  ài, the fourth tone, so make it loud and clear.
You is 你. It is pronounce as 你, the third tone.

After saying I love you, you may wonder what she or he feels the same way. You can ask 'do you love me?', which is '你爱我吗？(pronunciation:  Nǐ ài wǒ ma ？)

吗 (ma) is a word used at the end of a question.