The rules for the position of nicht are in detail discussed in the chapter 15 past tenses. For now it should be enough to know that the nicht is at the end of the phrase.

Ich sehe ihn nicht. = I do not see him. Ich gebe es ihm nicht = I do not give it to him. Er schenkt den Kindern den Apfel nicht. = He does not give the apples to the children. Ich sage es ihm heute nicht. = I do not tell him today.

Is the verb in a compound tense the thing get's really a bit more complicated, the details you'll find - as mentioned before - in the chapter 15 past tenses, here just one example:

nicht is before the perfect participle

Ich habe ihn heute nicht gesehen. = I have not seen him today. Du hast heute noch nicht gelacht. = You have not laughed yet today.

nicht is directly after the verb sein

Ich bin nicht reich. = I am not rich. Ich bin nicht so groß wie du. = I am not as tall as you are.

nicht is before the adverb

Ich esse nicht viel. = I do not eat much. Ich mag es nicht besonders. = I do not especially like it.