Much simplier:

declare @ThisDate datetime;

set @ThisDate = getdate();

select

getdate() as CurrentDateTime,

dateadd(mm, datediff(mm, 0, @ThisDate) + 1, 0) as BeginningOfNextMonth, -- Beginning of next month

dateadd(mm, datediff(mm, 0, @ThisDate) + 1, -1) as BeginningOfNextMonth -- End of this month

You can find some more date routine here Some Common Date Routines