Open up a new document, press first frame, F9, type:
trace("Hello World!");
Then Ctrl + Enter.
Next, try something like:
var str:String = "Hello World!";
var i:uint = 5
trace(str);
trace(i);
trace(++i);
trace(i + 5);
trace(i * 5);
trace(i /= 5);
trace(i);
You will see that it is very similar to Java in syntax and basic methods.