I found in a book from 2015 a box with the quote:
The beginning is half of every action. (Greek proverb)
I googled it and there are many "pop websites" with the same quote. But none with a reference or source. I found this site citing from Aristotle's The Nicomachean Ethics:
The beginning seems to be more than half of the whole.
Which is quite different (and in any case not a real "proverb"). Are you aware of such Greek proverb? Maybe it is a no-classical Greek (I haven't seen an explicit assertion of the contrary, to be honest). How can we tell whether it is legit or not?