tldr: rtfm!
(maybe read about the core principles of unix first. the links suggested by other commentators look good.)
learn how the documentation provided with your linux install is organized: man, info, apropos, whatis. the more comfortable you get with the built-in documentation, the more you'll find that most of your questions will be answered there. it takes a while to get used to it though. understanding the structure of the docs helps a lot with where to start reading. (did you know that the man page browser has a built-in string search?)
next step: dig into online documentation resources. start with the material curated by the distro project of your choice. don't forget wikipedia, some of their free software articles are nicely structured, concise and very well interconnected. you'll find plenty of useful links there, too.
third step: learn how to get the questions you can't answer yourself solved quickly by the community. "how to ask questions the smart way" by esr comes into mind. the way you asked your question here suggests that you might have read that already. anyway: for almost every question there are the right persons to ask and the right way to do so. mailing lists are good, but may not be fast enough. on the other hand, irc culture is a little rough sometimes, not everybody is a catalyst on freenode. look that up, there's a nice article about how to communicate on irc. if you know how to ask in which channel, irc is a fast resource.
when you get used to the culture of documentation and community, the gap between user and developer gets really small.
linux is transparent. learn how this transparency is organized and communicated.
that might sound easy, but it _is_ the hard way.
if you like the experience, consider writing on the web about your very own way of learning linux. or put some time into improving the resources you were using. that's a fun way to say thank you. :-)
have fun!
.~.