You've already learned about one ``modifier key'' in Emacs, the key. There is a second one, called the Meta key, which is used almost as frequently. However, not all keyboards have their Meta key in the same place, and some don't have one at all. The first thing you need to do is find where your Meta key is located. Chances are, your keyboard's keys are also Meta keys, if you are using an IBM PC or other another keyboard that has an key.
The way to test this is to hold down a key that you think might be a Meta key and type ``x''. If you see a little prompt appear in the minibuffer (like this: M-x) then you've found it. To get rid of the prompt and go back to your Emacs buffer, type C-g.
If you didn't get a prompt, then there is still one solution. You can use the key as a Meta key. But instead of holding it down while you type the next letter, you have to tap it and release it quickly, and then type the letter. This method will work whether or not you have a real Meta key, so it's the safest way to go. Try tapping and then typing ``x'' now. You should get that tiny prompt again. Just use C-g to make it go away. C-g is the general way in Emacs to quit out of something you don't mean to be in. It usually beeps annoyingly at you to let you know that you have interrupted something, but that's fine, since that's what you intended to do if you typed C-g!
The notation M-x is analogous to C-x (substitute any character for ``x''). If you have found a real Meta key, use that, otherwise just use the key. I will simply write M-x and you'll have to use your own Meta key.