Create USER and LOGIN permissions What permission should I give to a user or role so that it can Create Login or Create User in database.
As pointed above by Jeremy, permissions can almost always be found on the corresponding reference document within Books Online for SQL Server.  On these reference documents, there is typically a section titled Permissions (typically towards the bottom of the document) which contains your desired data.
CREATE LOGIN (copied directly from the reference)

Requires ALTER ANY LOGIN or ALTER LOGIN permission on the server.
If the CREDENTIAL option is used, also requires ALTER ANY CREDENTIAL permission on the server.

CREATE USER (copied directly from the reference)

Requires ALTER ANY USER permission on the database.