You should look in to the format that you are having your Rest service accept and add "x-www-form-urlencoded" if you are going to continue using it to send payloads to the URL.
You specify the Content-Type under the Headers tab in the Apigee console, you just need to look in to what the Content-Type the server accepts.
To pass parameters in the url you just attach ?
to the end of the Url and put the variable names and values. These are usually called queryStrings
like so:
www.example.com?name=Dman100&Rep=104.
This example will pass to variables name and Rep. When passing variables in the URL you must be careful to html url encode the variables and their values.
For Posts you send the content in the payload/body. In whatever format is necessary.