Check your computer to see if you are using the US international English keyboard. The latest operating systems already come with it installed, so then all you have to do is click on one of the five punctuation keys ` ~ ^ ' " , then the letter to be modified. Examples: ' + a = á, " + u = ü, among others. You can also use the right alt (or AltChar) key + another key: right alt + / = ¿, + 1 = ¡ , + e = é, and so on.
Or, you can also just use ALT plus numbers by following these simple codes. Just make sure the the Num Lock on the keypad at the right of your keyboard is on. Hold down the ALT key while you type the following combinations on the keypad. The character you want will appear when you lift the ALT key and finish typing the numbers.
ALT 160 = á
ALT 161 = í
ALT 162 = ó
ALT 163 = ú
ALT 164 = ñ
ALT 168 = ¿
ALT 130 = é
ALT 173 = ¡
ALT 129 = ü
For Macintosh, press the key combination option-e (press and hold the option key, and then press the "e" key). Release both, and then press the vowel that you want to have an accent.
Option-e+a = á
Option-e+e = é
Option-e+i = í
Option-e+o = ó
Option-e+u = ú
Option-n+n = ñ
Option-u+u = ü
Option-1 = ¡
Shift-Option-?= ¿