Simple Trigger : If you try to Delete a record, trigger will throw an error you can't do it.

trigger TriggerCanddel on Candidate__c (before delete)
{
   list<Candidate__c> del = trigger.old;
   for(Candidate__C c : del)
   {
       c.adderror('please donot delete the record');
   }
 
}

1. Trigger Name : TriggerCanddel
2. Checking with Candidate__c - a custom object.
3. Context variable : (before delete should have trigger.old ) delete for checking the old record if already present in Salesforce Candidate__c object.
4. Collection : List for holding all the records.
5. adderror : for throwing the error statements.

Soon to have a detailed descriptions on triggers.