“Begin to be now what you will be hereafter.”

Song lyrics, Hounds of Love (1985), The Ninth Wave

Source: Gone with the Wind

Attributed to Goethe by popular British novelist Marie Corelli in her essay "The Spirit of Work" as published in The Queen's Christmas carol : an anthology of poems, stories, essays, drawings and music / by British authors, artists and composers in 1905 by The Daily Mail of London.
Attributed to Goethe by William Hutchinson Murray, in his book The Scottish Himalayan Expedition (1951), this has been shown to be a misattribution at "German Myth 12: The Famous 'Goethe' Quotation", Answer.com http://german.about.com/library/blgermyth12.htm and "Popular Quotes: Commitment", Goethe Society of North America http://www.goethesociety.org/pages/quotescom.html
Misattributed
Variant: Whatever you can do, or dream you can, begin it. Boldness has genius, power, and magic in it. Begin it now.

Treason (1988)

Source: Let Me be a Woman

On deep meditation and enlightenment that transcends temporal experiences and most notions of selfhoody
Alan Watts Teaches Meditation (1992)
Context: [Successful meditation brings about realizations:] That we are no longer this poor little stranger and afraid in a world it never made. But that you are this universe and you are creating it in every moment... Because you see it starts now, it didn't begin in the past, there was no past. See, if the universe began in the past when that happened it was now; see, but it's still now — and the universe is still beginning now, and it's trailing off like the wake of a ship from now, and that wake fades out so does the past. You can look back there to explain things, but the explanation disappears. You'll never find it there.... Things are not explained by the past, they are explained by what Happens Now. That Creates the past, and it begins here... That's the birth of responsibility.

Source: Extremely Loud and Incredibly Close

Variant: Real courage is when you know you're licked before you begin, but you begin anyway and see it through no matter what.
Source: To Kill a Mockingbird