half the battle
A successful beginning, as in You've got the shopping list done—that's half the battle. This expression is an abbreviation of an 18th-century proverb, “The first blow is half the battle.”
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.