Well, it all depends on a context and situation, but plainly said, you use "could" when you want to ask someone for something politely and "can" is used for expressing ability.
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Syamimi Rahimwhen to use could you and can you ... asking for helpYou can use either one. It depends on who you are asking for help.
Syamimi Rahim"I can get you one of those"or"I could get you one of those"I can get ... ~ The speaker is completely willing to get you one; the speaker would probably like to get you one. He certainly knows how to get you one, and he can probably get you one very easily.