With the UK and US keyboard, you can get a TM (trademark symbol) by typing Shift-Option-2. You can get a (c) (copyright symbol) by typing Option-g. You can get a (r) (registered symbol) by typing Option-r.
If you want to find other special characters, or if you use a different keyboard layout, you need to do the following. Open the System Preferences and go to the International pane. Select the Input Menu tab, check the "Show input menu in menu bar" option at the bottom of the pane and then check the "Keyboard Viewer" option near the top of the list.
You will now be able to turn on the keyboard viewer by going to the International menu bar option (it will be the flag of your input language) where you can see the available characters. Try holding Shift, Option and Shift-Option to see other available characters.
