If you want to negate a German sentence, you need the word "nicht".
You put this at the end of the sentence.

Please note:
If the verb of the sentence consists of two verb parts, "not" is in the penultimate position, i.e. before the second verb part.
Example:
Bei Vollmond schlafe ich nicht.
I don‘t sleep when there is a full moon.
 
Bei Vollmond kann ich nicht einschlafen.
I can't fall asleep when there is a full moon.

Negating a German noun:

Negating a noun, using the definite article

If the noun you want to negate is preceded by the definite article, you need the help of the word 'nicht' (not).
You put this word in front of the definite article:
Example:
Ist das der Richter, der dich verurteilt hat? – Nein, das ist nicht der Richter.
Is that the judge who convicted you? - No, that is not the judge.

Negating a noun, using the indefinite article

For the negation of nouns with indefinite articles, you need the German 'negation article'.
These are declined as follows:

Please be careful when using a double negation:

If you deny a statement twice in German as in English, it becomes an affirmation!
Example:
Sie hat kein Interesse daran, an der Verlosung nicht teilzunehmen.
→ Sie hat Interesse daran, an der Verlosung teilzunehmen.
She has no interest in not taking part in the raffle.
→ She is interested in taking part in the raffle.