How do you spell no in Spanish?

"No" in English is also "no" in Spanish.

The pronunciation is however different, English (especially the American dialects) have "diphthongs" for most vowels (a gliding between two vowel sounds - in "no" the glide is from an 'o' to a 'w') but Spanish (many dialects) do not have these diphthongs to the same extent and certainly not in their "no". The Spanish 'o' sound is often shorter and constant (clear), no glide.