Negation in German splits into two completely different tools depending on what you are negating, and that is the first thing to rewire in your thinking if you are used to a single all-purpose negation word.
The first tool is nicht, used to negate verbs, as in Ich arbeite nicht (I don't work); adjectives, as in Das ist nicht gut (that's not good); and nouns that already have a definite article or a possessive, as in Das ist nicht mein Auto. The second tool is kein, used exclusively to negate indefinite nouns — nouns that would have taken ein or eine in a positive sentence, or nouns with no article at all, especially plurals. In fact kein is really just ein with negation built in, so it behaves exactly like ein: keinen for masculine in the accusative, keine for feminine and plural, kein for neuter and masculine in the nominative.
Here's the practical test: first check whether the positive version of the sentence would have used ein or eine. If yes, negate with kein/keine. If there was never an ein to begin with — a verb, an adjective, or a definite noun — negate with nicht.
This lesson also introduces the verb haben (to have), one of the most important verbs in German because it expresses possession, age, hunger, and much more, as in Ich habe Zeit or Ich habe Hunger. Its conjugation is slightly irregular: ich habe, du hast, er/sie/es hat, wir haben, ihr habt, sie/Sie haben — notice how the b disappears in du hast and er hat.
One last important point: when stating your profession in German, you never use an indefinite article. You say Ich bin Lehrer (I am a teacher), not Ich bin ein Lehrer. If your own language also drops the article before a profession, lean on that instinct here rather than translating word for word from English.
A very common mistake is reaching for nicht in every situation because it feels like the universal negator, producing Ich habe nicht Auto instead of the correct Ich habe kein Auto. Auto is an indefinite noun that would have taken ein in the positive sentence, so it needs kein, not nicht.