Do functions always need some sort of data going into them?
No. It is common to have functions that don't need any parameters. For instance, a function could return the current date, or the position of a movie clip on the screen.
Can I perform an operation between two numbers, if one is an integer and the other a float?
Yes. Flash allows you to mix integers and floats in calculations. The result will be a float.
If I compare two strings, will upper- and lowercase make a difference?
Yes. Flash considers uppercase letters to come before lowercase letters. This is not true in some other programming languages.
What if I create a loop that never ends? What happens?
This is called an infinite loop. It is a bug of the worst kind. Flash will detect that the program is taking an abnormally long time to run and ask whether you want to stop the program. It is then up to you to fix the problem with the loop.