This is a cool thread:
http://www.amiga.org/forums/showthread.php?t=19395It talks of replacing the double density floppy drive with a compact flash card reader!
I'd say AMOS is pretty ropey for decent games, I've only ever played one great AMOS game (it featured a 2-player game of biplanes bombing each others bases).
Blitz Basic 2 to start with, C then Assembly when you're more experienced. Skidmarks and Worms were both commercial games programmed in Blitz Basic 2.
I think DevPac was used for some games (a C compiler).
A cool Amiga to program for (and one that would give you massive street cred in the underground console coding world) would be the Amiga CD32.
Do the coding on an A1200, burn the CDR and attempt to play it on the CD32. You could even make a debug kit with an SX32(/Pro). Of course, you'd be limited to AGA but I'd say most Amigas floating around as games machines have only classic GFX chips.
MakeCD (the Amiga CD burning software) offers facilities to auto-boot CDTV (A500 console) and CD32 (A1200 console) discs.
Distribution could be achieved by approaching the German distribution companies who might be impressed by your software and put it on their website (maybe even print a box/manual). They also like selling just a keyfile over the net to a limited edition on their site or uploaded to Aminet.
I've wanted to make a game (even if it's a pacman clone) since buying my A1200 in '94. Maybe I'll get into it soon...
:-)