Doris,

I think your first web request is just receiving a token which you can then use to access a REST API endpoint.  I haven't done a Salesforce integration so I'm not familiar with how their authentication is set up.  Here's an example of how I got an InsureSign API to function (note: I'm not advocating for InsureSign - I ended up finishing this project with DocuSign which works the same way).  Hopefully, you can apply this to Salesforce.

There's a couple things to note here.  First I've circled the gear icon in red.  This gives you access to extra settings for the web request.  Second, I'm using an authorization header to get my access token.  If your client id/client secret requests gets you an authorization token, stick with that.  If not, you will likely need to log into Salesforce and figure out how to generate your authorization code.  If I remember correctly, the code I used was a hash of the clent id and client secret that I received by logging in to my account and setting up my api.

For some reason, I used a toolkit script to parse out my access token.  I think the ReadJSON activity as you have it should handle this just fine.  In fact, it looks like you already have an "access_token" configured.  Assuming you have an access token working properly, it is time to use it to retrieve information:

This request just retrieves a list of documents.  Note that I'm passing my access token into the header for the request.  I'm also specifying the /documents endpoint in the URI.  The result of the request is information about the documents in JSON format.  You would use a similar command with Salesforce.  Here's an example (I of course don't know if this is the API you are using or if this is the correct endpoint as I'm unfamiliar with the API): REST API Developer Guide | Get Field Values from a Standard Object Record (salesforce.com)

If you aren't familiar with Postman, it is worth looking into.  Postman lets you set up and test REST APIs.  With Docusign, all I had to do was configure the Postman environment for the Docusign collection and all the command examples just ran.  Like Docusign, they probably have an extensive collection of Salesforce examples already put together. It is a good way to test things out before implementing the workflow activities.