Where did Thanos get his first Infinity Stone?
In the beginning of Avengers: Infinity War, Thanos had one stone but where did he get it?
Does anyone know?
Thor explains it when he's rescued by the Guardians of the Galaxy:
He got the Power stone last week when he decimated Xandar and he got the Space stone from me when he slaughtered half my people
Thanos attacking Xandar (home planet of the Nova Corps, and where the Guardians left the Orb/Power stone at the end of Guardians of the Galaxy Volume 1) is just not shown in the movie.
Dave answered it quite well but here are the director's own words:
Speaking at Iowa City High, Joe Russo explained that the directors thought it would be "one too many" - that it would cause the writers to "get into a trap," where the film just became repetitive and predictable. Given Guardians of the Galaxy had clearly established where the Power Stone was, it was "easy to deal with it off screen." - businessinsider
So it just happens off-screen to avoid film looking repetitive.