1.To convert a casual way "だけど" into a formal way should be "ですけど".
Meaning is "but" and you can use it when you want to connect a sentence to another.
ex: えいがに いくんですけど、あなたも きますか?
(we are going to catch a flick, but do you want to come with us?)
2. To expect the listener to say something after that.
ex: A: のどが かわいたんですけど..... (I'm thirsty.....)
B: きっさてんに いきましょうか? (Do you want to stop by a coffee shop?)
3. When you are not sure exactly what you should do or what you should say next, and want to make it vague.
ex: まぁ、それでも いいですけど..... (Well, that's fine for me though.....)
Nowadays, Japanese people tend to add "だけど" or "ですけど" at the end of the sentence, but actually there is no such a big meaning.