You can follow the below step by step solution given in the below link. Link:- https://www.appseconnect.com/how-to-connect-salesforce-and-postman-using-rest-api/
In case you find any other issue please mention. If you find your Solution than mark as this as a best answer.
See below.
Similarly, for updates, you can use this: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_update_fields.htm
You can follow the below step by step solution given in the below link.
Link:- https://www.appseconnect.com/how-to-connect-salesforce-and-postman-using-rest-api/
In case you find any other issue please mention.
If you find your Solution than mark as this as a best answer.
Thanks and Regards
Suraj Tripathi.