Question

In addition to the comment below, some people may take these literally.

"Can you help me?" could be taken to mean "are you able to help me?" and the person being asked can say yes or no without having to action their reponse. They answered your question and that's it.

"Could you help me?" could also be taken to mean the same thing but people are more likely to be receptive to your asking for help, and actually help you.

That said, very few people are pedants over small things like that, and while 'can you help' is a bit more informal, both are fine and widely interchangeable.

Question

'Can you help' is less formal than 'could you help me'. More formal is: May I ask for your help, please?

The usage depends on the level of communication:

upward communication --> superior (boss), customers, acquaintances --> May I ask for your help, please? OR Could you help me, please?

parallel communication --> colleagues, friends, family --> Can/could you help me, please?

downward communication (not recommended) --> subordinates --> Can you help me?

I hope this helps.