The C-64 Programmer's Reference Guide (a.k.a The Bible) is a must of course, available online, and the http://codebase64.org/
is a decent resource too and if you're a bit dedicated this is enough to get you started. The forums at http://pouet.net/
and in particular http://csdb.c64.org/
and #c-64 on IRCNet together hold the bulk of the people making up the C-64 demo scene today I think, and are your best bets for expert help and info on the very intricate details of the hardware.
EDIT: as for the C-64 being difficult to program, some things are very straight forward on it being more simple than the Amiga, but in general going from the C-64 to the Amiga myself I can best describe the C-64 as being very fiddly to program because of its 8-bit CPU and the limited graphics hardware.