Trigger For prevent inserting data

i need a trigger that prevent some data from inserting in table According to a condition , before inserting any row in the table like this code

1. 
   

   create Trigger trigger_name on

   
   


2. 
   

   table_name for

   
   


3. insert


4. AS


5. Begin


6. if (true)


7. insert into table_name values ()


8. End