The direct train from Guangzhou to HongKong takes 1.5h with a fare of CNY190. That's fast and expensive. Alternative: take train from Guangzhou to Shenzhen which takes 1 hour and 10 minutes with a fare of CNY75. And then take east rail at Luohu which takes 30 minuts to downtown of HongKong with a fare of CNY33. That's accessible to every stop of HK and less expensive. You may choose this one.
For how many days to explore HK, i must say that 1 day is far from enough. 5 to 7 days is suggested if possible. Or at least 3 days.

Hi, I will be going Guangzhou next month. Can I know from Mr Newman where can I take the train from Guangzhou to Shenzhen? Thank you