To negate depends on what tense is written the sentence.
Simple present negative form: Use "don't" after (I, You, They, We) Ex: I don't like bananas.
And use "doesn't" after (He, She, It) Ex: She doesn't have English class on Saturdays.
Present progressive negative: Use "not" after verb to be. Ex: Laura is not playing tennis now.
I am not... You are not...
Past simple: Use "didn't" after all the subject pronouns. (I didn't... You didn't... He/She/It didn't... They/ We didn't and the verb in main form.
There are more tenses to explain but I don't time. I hope you find much more informations.