Just reiterating what Rao said. You intend to use soapui...do you mean the open source version or the licensed ReadyAPI! tool?
In regards to your description, you say you have a REST API to test....it uses a POST method and the payload of the request is in .json type and the response has a .json payload too. You want to assert against the content of the response's payload.
This all sounds typical stuff....does all my above description sound correct?
So what exactly is the question you want answering?
Also, you state you attached a file, but there wasnt any attachment in your post...?? Did i misunderstand?
if this helped answer the post, could you please mark it as 'solved'? Also if you consider whether the title of your post is relevant? Perhaps if the post is solved, it might make sense to update the Subject header field of the post to something more descriptive? This will help people when searching for problems. Ta