Well begun is half done; dare to be wise; begin!

[Dimidium facti qui coepit habet; sapere aude;
incipe!]

Horace (65-8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Epistles [Epistularum, Letters], Book 1, ep. 2 “To Lollius,” l. 40ff (1.2.40-41) (20 BC) [tr. Fairclough (Loeb) (1926)]
    (Source)

See Aristotle.

(Source (Latin)). Other translations:

The facte begun, to one halfe is done, be wyse and take good harte:
Begin.
[tr. Drant (1567)]

Begin, be bold, and venture to be wise.
[tr. Cowley (17th C)]

Set forth, thou'rt half thy way.
Dare to be wise: Begin.
[tr. Fanshawe; ed. Brome (1666)]

Begin: 'tis half the work: assume the power
To live: expect not for a fairer hower.
[tr. "Dr. W."; ed. Brome (1666)]

He that hath once begun a good design,
Hath finish't half; dare to be wise, begin.
[tr. Creech (1684)]

Dare to be wise; begin; for, once begun,
Your task is easy; half the work is done.
[tr. Francis (1747)]

In virtue's race to start is half to win;
Come then, be wise -- take courage and begin!
[tr. Howes (1845)]

He has half the deed done, who has made a beginning. Boldly undertake the study of true wisdom: begin it forthwith.
[tr. Smart/Buckley (1853)]

Come now, have courage to be wise: begin:
You're halfway over when you once plunge in.
[tr. Conington (1874)]

Make a good start, and you are sure to win.
So, then, have courage to be wise! Begin!
[tr. Martin (1881)]

Whoever has begun a deed, by so doing has accomplished half of it. Dare to be wise. Begin.
[tr. Elgood (1893)]

Well begun
Is half done. Dare to be wise. Get under way!
[tr. Palmer Bovie (1959)]

Once you start, it's nearly done. Be brave and wise:
Begin.
[tr. Fuchs (1977)]

To begin a job is half the labor: be brave
Enough to be wise: begin!
[tr. Raffel (1983)]

Get yourself going and you'll be halfway there;
Dare to be wise; get started.
[tr. Ferry (2001)]

Well begun is half done. Dare to be wise.
Start now.
[tr. Rudd (2005 ed.)]

Who’s started has half finished: dare to be wise: begin!
[tr. Kline (2015)]

He who has begun has half done. Dare to be wise; begin!
[Bartlett's]