If you have the Input menu enabled [In System Preferences > International] you can select "Show Keyboard Viewer." This will place a palette with a diagram of your keyboard on the screen.
If you hold down a modifier key (Like Option or Shift) or a combination of modifiers (control-option) they letters on the palette will change to show the characters that appear when you type a letter when holding down a modifier key.
For example, hold down option, and the 's' turns into a ß. That means that option-s produces a ß.
Some keys will have a white highlighting box drawn around them, namely ¨ˆ´˜ and `. These are symbols that will appear over other letters, and must be typed in two steps: first the symbol, then the letter you want under it.. The ¨, for exampke, is on the 'u' key and is highlighted. So to type the ö in your username, you'd have to type option-u and then 'o'. To get an ü, it's option-u and then 'u'. For an ó, the ´ is on the e, so it's option-e and then o. Try it,