Exec using Temporary table

use #tTable instead of @tTable
then

declare @str varchar(max);
set @str='select * from #tTable ' 
exec(@str)

first thing use select @set at the place of exec(@set) and select * from @tTable return more than one column so we can specify the particular column here...
And one more important thing execute the all statements in one batch because table variables have present only in a single batch.
i am giving the statements..

DECLARE @tTable table(ID INT,UserName VARCHAR(20),Password VARCHAR(100),Email VARCHAR(30))

INSERT INTO @tTable SELECT * FROM Login

DECLARE @set VARCHAR(MAX)
SET @set=(SELECT [UserName] FROM @tTable WHERE ID='2')
SELECT @set

declare  @tTable table(PeriodId int, Id int,Week varchar(50) ,res varchar(1000))

insert into  @tTable SELECT  PeriodId,Id, Week, res   FROM  mytable

select * from @tTable //Correct method to select data from @tTable 

In below code written by you is not in proper way because here @str is a variable not any object so you can't be able to access the data and stored whole query data because it's store only single value and "exec" it's a method to execute the stored procedure and worked for physical object

//Wrong way to select data from @tTable
declare @str varchar(max);
set @str='select * from'+ @tTable
exec(@str)

I hope you understand, what I'm trying to say.

Thank You......:-)