Should I abbreviate word starting with “kn” as “K” or “N”?

Abbreviations are usually based on the first letters of the words they represent, and I would choose "SK" in this case (additionally "SN" is a well-known abbreviation for "Serial Number" across many different industries, so it's probably a good one to avoid when possible as well).

Consider also the word "Herbs," a word for which some people don't pronounce the letter "H" -- regardless of how this word is pronounced, an abbreviation would normally include the letter "H" to represent it (using the letter "E" would be confusing).

Kn starting a word would become K, by convention. For instance, JSYK is common shorthand for Just so you know.

AFAIK, know is often abbreviated to K.

More generally, both phonetic and spelling-based abbreviations exist.