Where do I define the instance ID to start?

I haven't found an answer for this yet, and I've tried putting the instance ID I want to turn off/on:

ec2 = aws.createEC2Client(yourAccessKeyId, yourSecretAccessKey);

ec2.call("StartInstances", {}, function(err, result) {
  console.log(result);
})

Where in that script do I need to define the instance I want to start?

As Joe pointed out in his comment, startInstances takes an object parameter param which has to have an InstanceIds property. See more in the docs for the AWS SDK for Node.js.