Direct speech is when something is written or said exactly how the person who originally said it said it.
The English language is limitless and you could come up with lots of ways to rewrite those sentences. However, here is how I would rewrite your sentences.
1) "You have not paid your bill," he reminded us.
2) "I won't pay the bill unless the manager agrees to reduce the amount," he said.
3) "Don't worry, I will clean the house thoroughly," she said.
While doing so, I ensured to keep it as close to the original sentence as possible. However, you could make the sentences more creative by adding different words such as "declared" instead of said, or using words that make sentences flow more naturally (instead of reduce the amount, I would say "cut the cost"). It mostly depends on how close you are expected to keep it to the original sentences.