Hi Luis. 'Shall' is often used to express the idea of future in the first person singular and plural in the same way that 'will' is used for the other persons. But there is no future tense in English where the verb form changes. It can also be used in the interrogative form to suggest - Let's (Let us) do something as in: Shall we go out for dinner? 'Should' on the other hand has the sense of obligation as in: We should help those people who are less fortunate than we are.

"Should" is used to express something that you know you must do. For example: "I should do my homework tonight." "I should have done my homework last night." "Shall" is used to express the expectation that something will definitely happen. It is usually only used in the first person, and is similar to "will", except it's more formal. E.g. "I shall go there tomorrow."