I know it's the compose key, and I set it to right Alt and I set the "3rd level" option to the Shift + Alt, but I can't figure out for the life of me for example how to do the copyright symbol, or the registered mark, or trademark symbol, etc. I try alt then c but I get č.
Compose,o,c will produce ©.
You can view the combinations in your locale's Compose file, e.g.
For those without a compose key configured (it isn't by default), the setting is hidden away at System > Preferences > Keyboard > Layout > Options... > Compose Key Position. I prefer to use the "menu" key as compose, but it's a matter of preference.
AltGr (Third-level modifier)
On a UK English layout, AltGr+Shift+c also produces ©.
The standard USA layout doesn't include many characters at the third level as US keyboards have a second Alt rather than AltGr.
The layouts "USA International (AltGr dead keys)" and "USA International" have © at AltGr+c. "USA International (AltGr dead keys)" would be better for most as the other turns the apostrophe into a dead key for the acute accent.
For an occasional unicode character there is always Ctrl+Shift+*unicode-value*
You can get any and all unicode values (Codepoints) from many places;
Just search for 'copyright' and take you pick..
To enter it in a Gnome window (like here), just type: Ctrl+Shift+00a9
Here are a couple more. Just type Ctrl+Shift where you see U+,
Here is a link for general interest (or boredom :)
For the copyright symbol, try this page . [Actually, do what @misterben says in his terrific answer. I'll leave this link as a curiosity.]
As a generalized answer, if you are typing something odd that you do not often use, do a search for a word containing the character, and cut and paste.
For instance, on the Wikipedia Linguistics page , go to any of the languages listed on the left column. This solution also works for special characters with the correct search.
For searching-and-copying rare chars, this is invaluable: