Gelbart's AMS article *Introduction to the Langlands program* is a fairly standard place to start, for a broad overview of what the global conjectures look like and why we're interested in them. There's also a book by the same title by Bernstein, Gelbart et al, which gives more detail. There are also some notes by Knapp that I don't seem to have anymore, but you should be able to find them easily enough. There's an awful lot of different topics that all converge in the "Langlands program" -- to understand "everything" you're going to have to have good command of a pretty intimidating list of topics -- so the best thing to do (at least, what I found to be the best) is try and get a broad overview while taking a lot for granted, and then learn more about the things that particularly interest you. However, if you by "Langlands program" you really just mean "Langlands reciprocity for Galois representations and the local correspondence" then those references will be just fine.

Once you've got a basic understanding of what the conjectures say (or at least the global conjecture for $GL_2$) the answer depends on what you mean by a "serious" introduction. If you mean some level of formality, but skipping over lots of technical details and proofs then the two references above should be fine. If you want to actually understand things at a technical level (and be able to follow the proofs where they exist) then you've got a lot of work to do. I don't really know much about the global conjectures at a technical level, but for the local conjectures a good start is Bushnell-Henniart's *Local langlands conjecture for $GL_2$*.