Question: I am very new in security testing.

It will be very helpful if any one can suggest open-source/free tools that can run Scans for security issues (E.G SQL Injection) on REST APIs which use JSON requests.

Some services also use OAUTH.

Thanks

Answer: 
For VBScript or ASP Classic, or for modeling VBA (Visual Basic for Applications) such as macros used in Microsoft Excel (but without specific Excel libraries), there is:

<http://VBFiddle.net>

I developed this website/script a few weeks ago specifically as a Thank you! to all of the contributors of Stack Exchange, Stack Overflow, and all of their universe of sites, and I find it very useful for quick prototyping or to show someone my VBScript code... for example with VBScript's RegEx engine which has some quirks specific to VBScript.

VBFiddle.net does not yet support saving your VBScript on the website, but you can save your VBScript to say jSFiddle.net and link to that, and instruct people to copy and paste into <http://VBFiddle.net>. I will add saving support to VB Fiddle / Visual Basic Classic Fiddle if there is enough interest.

Enjoy!
Desktop editors like Context and Notepad++ provide syntax highlighters for Classic ASP. Microsoft Visual InterDev provides intellisense and code completion. It was part of Visual Studio 6. Unfortunately, current MSDN subscription provide access to VB6 but not Interdev. However old MSDN subscribers may have already obtained Interdev and be willing to sell it on Amazon. 

I am not aware of online editors.