“The secret of getting ahead is getting started.”

Commonly attributed to Twain in computer contexts and post-2000 inspirational books — the first sentence has also been attributed to Agatha Christie and Sally Berger.
Misattributed

Henry Allen (January 4, 1979) "Life in the Laugh Factory", The Washington Post, p. B1.

Variant: you don't have to be great to get started but you have to to get started to be great

From his autobiography http://nobelprize.org/nobel_prizes/physics/laureates/2000/kilby-autobio.html. The Nobel Prizes 2000, Editor Tore Frängsmyr, Nobel Foundation, Stockholm, 2001 <br class="br">Context: I&#x27;ve reached the age where young people frequently ask for my advice. All I can really say is that electronics is a fascinating field that I continue to find fulfilling. The field is still growing rapidly, and the opportunities that are ahead are at least as great as they were when I graduated from college. My advice is to get involved and get started.

Start All Over Again
Song lyrics, Enlightenment (1990)

Source: Vacuum Flowers (1987), Chapter 4, “Londongrad” (p. 50)

(p. 115 in The Hugo Winners vol. 1 edited by Isaac Asimov).
Short fiction, Exploration Team (1956)

My Life and Work (1922)

“If you can say it so calmly, then you haven’t.”
Source: Bitter Angels (2009), Chapter 11 (p. 155)