Article #000001016
Summary
Some guidance on connecting and developing for the Salesforce REST API.
Guidance
For testing out the API, use Salesforce Workbench. The online version is a toolkit from Salesforce. Log in using the credentials you will use for the integration, and then use the Rest Explorer TRY OUT tool within the workbench. You can connect, retrieve, Edit, and create records via REST API here. After logging in, go to the menu item UTILITIES >> REST EXPLORER.
You can find the tool here - https://workbench.developerforce.com/restExplorer.php
For using the REST API, please see the Developer Guide here:
https://appexchange.salesforce.com/appxListingDetail?listingId=a0N30000003HSXEEA4
Inserting vs. Updating
Perhaps stating the obvious, but for completeness, I should mention that if you are updating an existing record, you will first need to get the record you want to update. For contacts, something like an email address is unique enough to use. For matters, often a unique matter number is used. Both would be field values. In both cases, when you get the record you want to update, the true unique identifier for a table is the ID column, which is always an 18-character value.
You may want to update any contacts you create or update with your own unique external id for ease of integrating into that record in the future. If you want to do this, you would need to create a field of a special data type called "External ID," which is a text field. If you are creating records with your post, you are going to need to understand any other required fields to the contact, matter, or other table/object that follow their business rules for data quality.
Also, there is lots more information on the internet to help with this type of thing, google "update/get/post/etc." + " records Salesforce REST API" or similar search terms and you will get a host of results to help in design/troubleshooting/etc.