Dynamic SQL To Populate A Variable

Here's the WRONG way to do what I want. I need a way to populate a variable from the output of a dynamic query.

declare @TableName sysname
set @TableName = 'Customers'

delcare @Output bigint

declare @SQL varchar(max)

set @SQL = 'select top 1 RowID from ' + @TableName

select @Output =

EXEC (@SQL)

create function udf_GetDatabaseFingerPrint(

@DBID bigint
)
begin
returns bigint
as
declare @dbname sysname

, @iBig bigint
, @tSQL varchar(2000)
select @dbName = Name from Master.Dbo.Sysdatabases where DBID = @DBID
set @tSQL = 'select sum(Rows) from ' + @dbName + '.dbo.sysindexes'
set @iBig = exec(@tSQL)
return @iBig
end