I am using english Dvorak layout and a keyboard with US-layout. My OS is Debian. I want to print characters like: ç, á, à, ó, é, ú, í, and ä. How I do this?

You could change your keyboard everytime you need these characters. Or have them somewhere in a file and copy paste them. Or use the ultimate hold <alt> and enter some number, release alt and hope that you hit the right number of the character you want.

<ALT><6><5><0> brings è

Run the keyboard configuration tool from the menu. Where you can select the layout, there should be a button labeled "options". Go to that page and look for "Compose key". There you can assign a spare key (the Windows Menu key is a good choice) to be a compose key. You can read about it at Wikipedia, but the idea is that you press the compose key, a key representing the accent, and then the letter. So
<Compose><comma>c for ç
<Compose><apostrophe>e for é
<Compose><double quote>a for ä
and of course you can use the grave, circumflex, and tilde accents, as well as
<Compose><semicolon>e for ę
<Compose>ae for æ
<Compose>cc for č
<Compose>oa for å