If you have those multimedia keys, or some other special keys on your keyboard and you don't know what their keycodes are, then you can run an application called xev. When you run xev you should see a small window with a smaller square inside of it. While this window is up, you should see any keyboard (or mouse) event that you create scrolling inside the terminal from which you started xev. Here is an example entry from xev output for the little internet looking button on my laptop.

KeyPress event, serial 30, synthetic NO, window 0x1400001,
	root 0x4c, subw 0x1400002, time 233224347, (42,43), root:(512,536),
	state 0x0, keycode 178 (keysym 0x0, NoSymbol), same_screen YES,
	XLookupString gives 0 bytes: 
	XmbLookupString gives 0 bytes: 
	XFilterEvent returns: False

We are only interested in the "keycode" part. It looks like the keycode is 178 for my internet button.

Now that we know what the keycode is we want to map some command to that keypress. First, we need to use xmodmap to register a keysym to that keycode. To do this we execute the command

xmodmap -e "keycode 178 = XF86WWW"

If you want to see a list of valid keysyms, look for a file named XKeysymDB. It is usually in /usr/share/X11/, but this may differ on your distro. Find it with "locate XKeysymDB".