That depends on which application you are using.
In vim, add this to your .vimrc
Then, when writing a document, escape out of insert mode, and enter :digraph at the command prompt to see the letter/key combinations for various special characters, such as the upside down question mark. The keys are pressed: first letter key, backspace, second letter key.
In OpenOffice.org Writer, Insert - Special Character, and select the character from the dialogue box that pops up.