1. Hey Alberto, let me tell you a secret, but 別說出去. (don't say it to anyone)
2. A: thank you Alberto so much for what you have done for me, i owe you my life!!!
B: 不會,別這樣說。(It's nothing, you don't need to mention it)← you can use this if you wanna be modest and don't wanna take credits to what you've done for others.
3. A: Oh Alberto, you look terrible, what happened?
B: 唉... 別說了。(Sigh, i don't wanna talk about it. it's painful to even think of the tragedy.)
i hope my examples are easy enough for you to understand. Chinese is hard!