I suppose you can always audit a course or work your way through an intro textbook.

Or, honestly - it's *not* a linguistics textbook, but if all you want to do is get a start on the subject, you can do worse than to read the online version of The Language Construction Kit. (There's also a print version and a sequel.) It's geared towards conlanging rather than a serious introduction to linguistics... but it's free, and written in an accessible way.