welcome to the wonderful, ever-expanding world of the Commodore 64
Cartridge-wise I strongly reccommend Pitstop II; a racing game that kicks serious ass (especially 2-player)
EPYX fast-load is almost a must-have with disk-drive (as CRT pointed out)
games can be hard to find in "good" condition; but lo' and behold, there is a solution
http://sta.c64.org/xm1541.html
with this you can make a cable which connects your 1541 (or other disk-drive) straight to your PC
then all you need to do is download the games and copy them onto 5.25" disks
it's a good way to ensure that you have backup's of your games, so that you don't *have to* use the original disks, since these can (and will) get older
I used only tapes (datasette?) from 1984-1992 and although it was a horrible experience, it got me through the most part of my C-64 days
mind you that there is a tiny screw in the datasette player; I'm not sure what it's called in English, but in Swedish it would translate to "tune-head"
this "tune-head" sometimes need to be changed in order to play different games, because some games (backups mainly) will run on a different tune-setting
getting that tune-head in proper place for all of your games is a true bitch
*but*
if you are running backups on tape and the game fails to load, then it is *most likely* the tune-head which needs adjustment
if you use a tape-loader (neccessary for most backups where there is more than one game/tape), I strongly recommend the ABC-Turbo Loader
the thing with this particular loader is that when you adjust the tune-head you can see the lines having more/less distance from each-other
when they are perfectly formed (as in the start of this video) then the tune-head is in perfect position
it's overly complicated and idiotic, hence I recommend using Floppies only;
but if you for some reason want to use tapes as well, then just send me a PM, or similar if you get issues
AFAIK "most" people only used floppies for the C-64, hence it can be a bitch getting help with the datasette, but I am fairly experienced with it =)
there is also an OS for the C-64 called "GEOS"
this contains
basically the same stuff as early, early Windows
it is
not required to run any games, albeit it does have a paint-program, word-processor, etc.
the one game that every C-64 owner should get is (of course) The Great Giana Sisters, which is a European (German, IIRC) "clone" of Super Mario Bros. which (if you ask the "computer-nerds") far surpasses SMB
it is my opinion that it may not be "better" than SMB, but it is definitively in the same league
and however much I love (LOVE) Wico joysticks (Wico Commander comes to mind), I strongly recommend getting a couple of TAC-2's as it was voted the best joystick for several years in a row
they are very reminiscent of arcade-sticks and can handle a good beating
also as for more games to recommend...:
Monty Mole games
Dizzy games
Summer-games/Winter-games/World-games/The games/California-games
Delta
Pharao's Curse
Barbarian 1+2
Skate or Die!
Knights of Legend
Level 9 games (text-adventure games, in the same league as Infocom, albeit some have graphics, as well)
Boulder Dash (seriously. all of them. get them. there are *still* levels being made for it)
Maniac Mansion/Zak McKracken (LucasFilms early adventure games)
International Karate 1/2/+
Exploding Fist/Fist II/Fist+
Wizball
Rags to riches!!!!!!
Spindizzy
Chimera
Uridium
I can write a whole book on "must-have-games" for the C-64; but if you're interested just name a genre and I can see what I can shake out of mah sleeve
also
http://www.protovision-online.de/games/newcomer.htm
Newcomer, latest game to be released for the C-64
also, you will no doubt discover the wonderful "Demo-scene" of the C-64
this is (to most "today's C-64 users" the back-bone of the C-64)
Demo's are... well... pictures, music'n'stuff
here's an example:
and then of course you can learn BASIC and program... simple programs
or go on and learn Assembly-language, etc.
there is just so much to the C-64 that once you get hooked... it's hard to ever stop
congratulations on your purchase
I have no doubt that you will be very pleased with it