Savina has just asked a question about the use of "shall" or "could", as in the question, "How shall we get into the city centre?" (see her post on the Tag Board.)
The two words carry different meanings. If we ask, "How shall we get into the city centre?" ("to" the city centre is more correct), we are inviting a suggestion. "Shall we go by bus? Or shall we go by taxi?"
But if we ask, ""How could we get to the city centre?", we are discussing possibilities. "We could go by bus. Or it might be possible to go by taxi." The suggestion is that we are uncertain as to the available methods of getting to the city centre.
Both "shall" and "could" belong to that family of verbs known as 'modals'. For further information, check out the British Council's Learning English site. Here's a direct link to 'Modal Verbs (1)'.
Unfortunately, "shall" is not discussed here. To get an idea of how "shall" is often used, try the Virtual Language Centre Concordancer to see examples. Type in "shall" as a search word, and see what you find. Then type in "could", and compare and contrast the differences between the two words.
Tell us about your findings in a Comment after this post. You can also try the Tag Board but there is a limit to how much text the Tag Board will accept.
Good luck and more soon!