Plan to go to Macau by ferry & return in same day. I stay at causeway bay, Hongkong

You can take the Island Line from Causeway Bay Station to Sheung Wan Station. Get out of the station, you can find the HK-Macau Ferry Terminal where you can find the ferries to Macau.
There is a ferry from 07:00 to 23:59 every 15 minutes. For economy seat, the fare is around 159HK$ for daily ferry, 172HK$ for weekends and holidays, 195HK$ for night ferries.

Thank you very much Jessica.