It depends what are you saying "no" to. For example:
- I'm sorry, but I can't go. 对不起,但是我没办法去。
- I'm sorry, but I have to handle something, and can't help you. 对不起,但是我有点事,不会帮你。
Usually, you need to say "I'm sorry" (对不起) and then give some reason for why you are saying no.
When it' s somehing simple like when they ask you If you want something in the shop or restaurant you can also say "no need, thanks" - “不用,谢谢”
- I'm sorry, but I can't go. 对不起,但是我没办法去。
- I'm sorry, but I have to handle something, and can't help you. 对不起,但是我有点事,不会帮你。
Usually, you need to say "I'm sorry" (对不起) and then give some reason for why you are saying no.
When it' s somehing simple like when they ask you If you want something in the shop or restaurant you can also say "no need, thanks" - “不用,谢谢”