我先谢谢了

先谢谢你啦

Jolena's answer in the comment section is completely wrong and I have never seen a person from Guangdong who speaks such terrible Mandarin. The other answers are very good. I would personally say "先谢了" on casual occasions.

A good question! 先 ：in advance 多谢: Thank you so much 你: you Mandarin style: 先多谢你！ *For extra learning: Cantonese style ： 多谢你先！ Cantonese usually put 先 at the end of a sentence.

先谢谢你啦。 or 提前谢谢你啦