As waiwai993 answered, Can/May/Will have different meanings. However asking someone if they can help you usually implies that you would like their help, and it gives the person an opportunity to decline without being rude. Maybe they are able to help in theory, but can't right now because they are too busy. In that case they can say that they can't help. If they simply don't want to help they can just decline without a reason. So I would always use "Can you help me with this" or "Could you help me with this" unless I needed a more specific case (such as I KNOW that they CAN, but I am forcing to answer whether they WILL or not, or I know that they are ABLE, but maybe their mother won't let them*).
