Question

can anyone tell me that why UNO was formed and when

Answer 1

formed on 24 October 1945.
after world war 2 to stop such wars.

Answer 2

After world war I,a need for a global body to ensure peace was felt.thus league of nation was established but the body failed to preventthe next world war.so after World WarII a new organisation was formed abolished the league and learning from the previous experiences.this body was UN.
The United Nations was officially formed in October 1945 to promote cooperation among different countries.It had its roots in the former League of Nations, which was formed after World War I but disbanded after failing to prevent the second World War.