才vs 先: When to use each one in Mandarin?
才 and 先 are both very common words in Mandarin. Like you said, 先 means first in Cantonese, but it is the same in Mandarin.


先 means first or previous in time(or order), or means earlier than ..., or means in advance, or means before ..., the opposite word is 后(means later).
examples: 早先,先前 both mean previously(or in the past). 先人 means ancestor

她比我们先到了那里——She got there before us.
才 has two kinds of character. 

When it's used as noun it means
ability or talent or gift. (example:你真有才——you really have talent! 多才多艺——be gifted in many ways.) 

When it's used as adv, it
means just now (example:刚才——just now, 才开始——just start), 

or means
things happen or finish so late (example:你怎么才来——Why are you so late!,
他昨天才走——He doesn't leave until yesterday). 

or means short in
number (example:你才十八岁——you are only 18 years old!) 

or means only
...when (example: Only when you try hard, you can get good
results——努力才有好结果), 

or mean emphasize (example: I'm not buying what you
said at all——我才不信你说的呢)




this is all I know, hope it helps