These are ALT-Codes. You use them to make special characters that your keyboard does not have. To use them you must have the NUMBER LOCK on, and use only the number pad on the right, not the numbers along the top of the keyboard. To write á; you hold down the ALT key while entering 0225 on the number pad.
I recommend printing this table and keeping it somewhere near your computer screen for easy reference. In time, the most common of these may become second nature to you, but I still need to look at the chart sometimes!
á - 0225 | â - 0226 | ä - 0228 |
é - 0233 | ê - 0234 | ë - 0235 |
í - 0237 | î - 0238 | ï - 0239 |
ó - 0243 | ô - 0244 | ö - 0246 |
ú - 0250 | û - 0251 | ü - 0252 |
ý - 0253 | ÿ - 0255 | |
Á - 0193 | Â - 0194 | Ä - 0196 |
É - 0201 | Ê - 0202 | Ë - 0203 |
Í - 0205 | Î - 0206 | Ï - 0207 |
Ó - 0211 | Ô - 0212 | Ö - 0214 |
Ú - 0218 | Û - 0219 | Ü - 0220 |
Ý - 0221 | þ - 0254 | |
Ñ - 0209 | ñ - 0241 | |
Œ - 0140 | œ - 0156 | |
Æ - 0198 | æ - 0230 |