How can we add a field in object using Salesforce trigger?

Hello Madhulika,
You can refer the following example:

1. Account (Parent Object)
2. Contact (Child Object).
3. Contact_Recs__c (Roll up summary field/Custom Field).
4. accountid (Lookup field).

Trigger Code:

trigger CountContactsnew on Contact (after insert, after delete, after undelete) {

List accIdList = new List();
if(Trigger.isInsert || Trigger.isUndelete){
For(Contact con1 : Trigger.new){
accIdList.add(con1.accountid);
}
}
if(Trigger.isDelete){
For(Contact con1 : Trigger.old){
accIdList.add(con1.accountid);
}
}
List accUpdateList = new List();
For(Account acc : [SELECT Contact_Recs__c,(SELECT id FROM Contacts) FROM Account WHERE id =: accIdList]){
acc.Contact_Recs__c = acc.Contacts.size();
accUpdateList.add(acc);
}
try{
update accUpdateList;
}Catch(Exception e){
System.debug('Exception :'+e.getMessage());
}
}
Thanks.