In a formal situation, or if you are asking help from someone that you don't know, use "could". It is less direct and more polite.
In an urgent situation, or if you are speaking to someone you know well, then the less formal 'can' is acceptable. It is more direct and less polite.
As fdmaxey inferred, try to use "please" when making a request...and don't forget "thank you" afterward.
One of the important functions of the modal verb 'could" is to express a polite request. Since verbs do not have polite forms in English, as they do in German, French, Russian, and many other languages, 'could' is a valuable 'tool' in the native English speaker's vocabulary.
A non-native speaker, who may not have a complete understanding of the English speaking culture, is wise to be polite. It also makes a good impression...something that many native English speakers seem to have forgotten.