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I hope this helps and in case if this comes handy can you please choose this as best answer so that it can be useful to others that might face the same issue in the future.
Additionally, Have you checked trailhead it has detailed description and exercise problems please have a look at the trailhead link below, although there are two you can practice them and after which you can build your own scenarios to implement using trigggers:
It looks like our team of experts can help you resolve this ticket. We have Salesforce global help-desk support and you can log a case and our Customer Success Agents will help you solve this issue. You can also speak to them on live chat. Click on the below link to contact our help-desk. Trust me it is a support service that we are offering for free!
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Thanks,
Jarvis SFDC team
I found this below link, can you please have a look at it.
>> http://www.sfdcmeet.com/development/apex-in-salesforce/apex-trigger-salesforce/
I hope this helps and in case if this comes handy can you please choose this as best answer so that it can be useful to others that might face the same issue in the future.
Regards,
Anutej
https://trailhead.salesforce.com/en/content/learn/modules/apex_triggers
Also, I would suggest you to learn triggers best practices while you are learning the implementation part.
Trigger.NewMap is only available in before update, after insert, after update and after undelete events
Please see below for detailed explaination: https://trailhead.salesforce.com/en/content/learn/modules/apex_triggers/apex_triggers_intro
Also, the below is not salesforce official documentation but can help
https://harshitcoder.blogspot.com/2017/08/all-trigger-examples.html