where to start rest api testing

Here is a good place to start.

http://readyapi.smartbear.com/soapui/rest/intro/start 

Using SOAPUI Pro you can run data driven tests, pass values between API calls with a right-click of a button.

Refactor for test maintenance, use composite projects for team testing .

You can then run Load performance and security scans re using the functioanl tests you have already created.

You can also use SOAP UI OS to test Rest APIs but wound't have any of the functionality mentioned above available to you.

See here for more info - https://www.soapui.org/rest-testing/getting-started.html 

Ronan