how can I put an emphasis above a letter - ie. in spanish

You can add spanish keyboard layout and change to it if necessary.

Go to System Preferences>Language and Text> Input Source tab. Then look at the list and search for Spanish and select it.

On that tab you have also option "Show Input menu in menu bar". Select this and then from top bar you will have an option to change keyboard layout when need it by clicking on the flag.

If you just want to type the letters, but not use a full Spanish keyboard, either press and hold the letter key, or use the key chording combinations.

For press and hold, after about a second, all the possible accents are shown in a window above the letter. Select one by number or by mouse.

For keyboard chording, hold down option and press the key that has the accent, then press the letter.

For instance, on the US keyboard, the ´ accent is on the e key. Press opt-e, then a to get á.

If you have turned on "Show Input menu in menu bar," select Keyboard viewer from the menu. Then, hold down the option key. The keyboard will show the chording keys highlighted in orange.

If you use the US Extended (or any Extended) keyboard, almost all of the possible language accents are implemented like that.