How can i pull current opportunity data and send to external third party service from salesforce

Question

I want to add a custom button on opportunity item and on click of it will fetch the opportunity contract details and send it to third party service. I read about the rest callout by which we can make the third party api call after register it on Remote Site Settings But how can we include the current opportunity items details with it?

HttpRequest request = new HttpRequest();

request.setMethod(method);
request.setEndpoint(endpoint);

HttpResponse response = http.send(request);