When do I use "nicht" and when do I use "kein"?
nicht: Negates verbs, adjectives, adverbs, proper nouns, definite articles, possessive pronouns, and entire sentences.
kein: Negates indefinite articles ("a/an") and nouns used without an article (e.g., uncountable nouns, plural nouns in general). "Kein" declines like "ein".
Example
Ich spreche nicht Deutsch. (Negates verb)
Das ist nicht schön. (Negates adjective)
Ich habe kein Auto. (Negates indefinite article "ein Auto")
Ich habe keine Zeit. (Negates noun without article "Zeit")