The Usenet News Group comp.os.linux.m68k is read by many Amiga users who use Linux. It's a good place to ask if you have simple questions about Linux that are specific to the Amiga (see below for Linux/APUS questions).
You can also ask more generic questions about the use of Linux even though you would probably be better off posting to groups with more traffic and readers. Remember, Linux is available on many platforms - an application problem you have is likely also to be a problem for, say, an ix86 (Intel) user.
And there are tons of help to be found in FAQs, HOWTOs and general Linux documentation (see Chapter 12). If you are new to Linux you might even consider buying a book about Linux usage - this is one of the things that sets Linux apart from Amiga DOS; there are so many users that there's commercial interest in supporting it (with books and hot-line help).