Diacriticals are easy on MacIntosh.  They use the OPTION key along with another (usually very logical) letter key, as follows:
 

• For an accent, you press OPTION and the letter e at the same time, then release them and type the vowel over which you want the accent to appear.  (The letter e is the most commonly accented letter.)

• For a diéresis (the two dots over the u in words like vergüenza,) it's OPTION and the letter u at the same time, then release and type u again.

• For the tilde (ñ), press OPTION and the letter n at the same time, then release and type n again.

• For the inverted question mark (¿), press OPTION, SHIFT, and ? all at the same time.

These commands work in any MacIntosh software. If you use these commands and get a little square where the special character should be, it means the font you're using doesn't support accents. All you'll have to do is change the font.  (However, very few fonts support accents over a capital letter. It's okay to leave out accents over capitals.)
 
 

For IBM PC and compatibles, you have two options: one involves typing in numeric codes (the best option if you're using public computers or someone else's computer) and the other method is by reconfiguring the software that controls your keyboard. Here is a page that gives instructions for both methods.