But firstly, seeing the "Hello world" prompt gets boring very quickly, you can either comment it out by adding // to the beginning of the line and it will be ignored, or you can just delete it.
#target indesign
//Now you know how to add comments to your code
//alert("Hello world");
app.documents.add();
Ok, back to saving and naming your file in one go. Just add
#target indesign
//Now you know how to add comments to your code
//alert("Hello world");
app.documents.add();
app.documents[0].save();
Hit F5 (Ctrl+R on a mac) to run it.
Notice the [0] attached to app.documents[0].save(); this is important as you may have many documents open so you need to tell Indesign to use document 0, which is how counting in javascript works, [0] equals item1, [1] equals item2, [2] equals item3, and so on.
Next, we'll create a text frame and add content to it.
