Pre:
1.?Set up Connected App in SF
2. Install Postman
3. Get SF access token(SFDC - Using Postman to get SF access token)
Steps:
1.??On Postman, choose type 'GET' and set link to this format:
your org link/services/data/v42.0/query/?q=query
eg. https://ap17.salesforce.com/services/data/v42.0/query/?q=SELECT+Subject+FROM+Case
2.?Click Headers and 'form-data' and set below key and value information:
? ? - Authorization: your token type+ access token

Postman GET request