There are several ways to do accent marks on the computer.  Some people copy and paste from the character map, others change the language to Brazilian Portuguese.  The latter, however does not accomplish the task, because most of the accents are not automatically placed over the accented vowels.

Here’s the method that I use.

Windows:

Make sure the number lock  is on.  Press and hold ALT, then enter the number corresponding to the letter.  When you lift ALT the letter will be inserted where the cursor was placed.

á – Alt 0225              ê – Alt 0234            É – Alt 0201

é – Alt 0233              ô – Alt 0244           Ó – Alt 0211

í – Alt 0237               õ – Alt 0245

ó – Alt 0243              ã – Alt 0227

ú – Alt 0250             ç – Alt 0231

Mac:

á, é, í, ó, ú        Option e, letter      ê – Option i, letter

ã, õ        Option n, letter                   ç – Option c, letter

With practice it gets easier.

Boa sorte!