special font (letters)

How can I reach special letters like an e with trema (2 dots on top)?

if you are on Mac, do the following:
ALT-U, followed by the E (to get the ë) or followed by SHIFT-E to get the Ë
As a hint, ALT-U for Umlaut (will also produce ü ö ä and Ü Ö Ä)

That’s wonderful, thank you. And what about le cédille and all that other stuff?

On Mac, you can type Alt+c for a cedilla. You might find this page helpful.