Below HTTP Request retrieves all the record's fullname field from contact entity, more comma separated fields can be added

GET [Organization URI]/api/data/v9.2/contacts?$select=fullname 
Accept: application/json  
OData-MaxVersion: 4.0  
OData-Version: 4.0
If-None-Match: null

Sample response of above query

{
    "@odata.context": "https://[Organization URI].crm8.dynamics.com/api/data/v9.2/$metadata#contacts(fullname)",
    "value": [
        {
            "@odata.etag": "W/\"867043\"",
            "fullname": "Yvonne McKay (sample)",
            "contactid": "fe2a6a27-89fd-e711-a836-000d3a33a7cb"
        },
        {
            "@odata.etag": "W/\"866666\"",
            "fullname": "अनामिका अग्रवाल (नमूना)",
            "contactid": "540b4637-a636-ec11-8c65-000d3af2b6a6"
        }
    ]
}

If all attributes need to be retrieved, remove $select clause like shown below.

GET [Organization URI]/api/data/v9.2/contacts
Accept: application/json  
OData-MaxVersion: 4.0  
OData-Version: 4.0
If-None-Match: null

Sample response of above query

{
    "@odata.context": "https://[Organization URI].crm8.dynamics.com/api/data/v9.2/$metadata#contacts",
    "value": [
        {
            "@odata.etag": "W/\"867043\"",
            "customertypecode": 1,
            "address2_addresstypecode": 1,
            "merged": false,
            "territorycode": 1
            ...
            ...
        }
    ]
}