In the most simplistic explanation:

Should (past of shall) implies the obligation or duty to do something - I should study, but I don't feel like it. 

Would (past of will) I told him I would be away for a few days.

Would also can express the conditional mood, indicating the consequence of a possible situation.  I would buy that car, but I don't have enough money. Or: I would help you if I had the time.

Could (past of can) expresses possibility - You could do that if you tried. Or: Could you help me?

Could and would can get confusing because they seem interchangeable at times, but the meaning of the sentence is different. For example, Could you help me? implies "Are you able to help me? whereas Would you help me? implies "Are you willing to help me?" An answer may be: "I would if I could." which means "I am not able to help you because I do not have the ability or means to do so.

confused about using should would could

confused about using should would could..
someone help me please.