Probably the most important thing you can do is simply play around
with the technology.  Get some ideas of what you want to play around
with.  Then try to do it or see if someone else has already done it.
If someone else has done it, try to figure out how (open source for
the win).

When I was starting out I liked having classes, just because they
usually create goals and end points.  To be honest though it's been a
little while since I've actually taken a class.  I probably should
again, but life does get busy.

Books and very good websites are a close second.   Look for classes in
either your CS department or the local community college.

If you want to do web development, start looking for a language and
framework you like.  Set up a box, install a webserver on it.  Find a
web application you like and try to get it up and running.  (Give a
try on doing something like running your own koha server!)

I don't know if it will help, but here's some knowledge I'd look for
in any web developer that was looking for a library job:

* What version control systems do they know?

* Do they know project management tools like puppet?

* Why they liked particular projects they worked on and what they may
not liked about them.

* Basic network knowledge.
* Some basic knowledge of design principles and usability testing.
They don't need to be a master, but I hope they're at least aware of
some the techniques.

I'm not really concerned about particular languages or frameworks

Mainly I'm looking for signs that they're comfortable with web
development and know some of the pitfalls and issues that can happen
in the library environment.  Have they run into issues with combining
diacritics,  confused librarian, what to call services?   Also, I'm
watching for any warning signs like like they can't distinguish
between client-side javascript & server-side processing or they only
seem to use "does it display".  That would make me instantly wary.

Jon Gorman