你好！我係美國人。我學廣東話係因為我想喺香港工作。

It's the way how Hong Kong people speak everyday, but that's not a formal writing, it's something more like a translation from talk to word. You can use this in WhatsApp or chat
 
But if you going to write formal , just thought about how to say this sentence in Mandarin then write it down. All HK people are writing in the same way as Mandarin except we are using traditional Chinese. Here is the example: 你好！我是美國人。我學廣東話是因為我想在香港工作。

你好! 新年快樂!

你好！我係美國人。我而在家學廣東話因為我想在香港工作。

BTW， English also works in HongKong, most of Hongkong people can understand English. 

你好! 新年快樂!

你好！我係美國人。我而家依家學紧廣東話因為我想係香港工作。

學紧 means learning

 

你好! 新年快樂!

你好！我係美國人。我而家學廣東話因為我想香港工作。