When do I use “can” or “could”?
I am a little bit confused about asking a question:
Can you please tell me my next work?
Could you please tell me my next work?
Which is more correct?
“Could you” is more polite than “can you”. Regarding the rest of the wording, something about “tell me my next work” rings wrong to my ears. Probably, because you don't “tell a work”. So, maybe “tell me what my next work is?”. Finally, in most context, I'd use “job” instead of “work”, but that may be just me!
They have slightly different emphases.
"Can you tell me..." means do you have the capability to tell me, for example, do you know what I should do next? The emphasis is on the capability of the person being questioned.
"Could you tell me ..." expresses possibility, "is it possible for you to tell me..." The emphasis is on the likelihood or the information being revealed.
In both cases the answer is not a work assignment ("you should do such and such") but yes or no. "Yes I can tell you" or "Yes I could tell you."
Most likely if it is a polite question, "could" would probably be more appropriate, though they are used pretty much interchangeably in casual speech. To answer just "yes" or "no" would be strictly correct, but again, in casual speech, to do so would be considered rude and pedantic.
If you are asking what your next work assignment is, I'd suggest you say "Please tell me my next assignment." That would convey your question politely and clearly.
Note also that asking for "your next work" is likely not correct, unless you are using a special meaning of work. You would have to ask for your next work item, or next piece of work or something like that. The way you phrased it is probably not correct.