How to isolate the result of SELECT's?

Hi,

I need to do an operation that I will use some SELECT's and get the
results, but I want to have sure that those tables have not been
changed with INSERT's or UPDATES during the operation.

Example:

BEGIN OPERATION
Select field from table1;
...
Select other_field from table2;
...
END OPERATION

How can I lock these tables to assure that the tables are not getting
INSERTS's or UPDATE's during the operation?

Best Regards,