Не goes directly before whatever it negates, usually the verb - я не работаю. Нет is different: it is the word for "no", and it also means "there isn't", in which case what is missing goes into the genitive - здесь нет кафе.
Examples
- Ещё не час.
- It isn't one o'clock yet.
- По воскресеньям я не работаю.
- On Sundays I don't work.
- Нет, я не говорю по-немецки.
- No, I don't speak German.
The mistake to avoid
Не and нет are not interchangeable: не negates a word, нет denies existence.
Where this fits in
Numbers, days, time, Russian A1: Count, talk about days and months, give the time, and handle numbers in real situations.