If you want to make games on your A1200 using the AGA chipset then Amos isn't going to help. Mattathias Basic could help, if it were finished, but it isn't ready yet. To get the latest version of Blitz Basic (called AmiBlitz) download it from
here but I don't think it has a manual available for it.
In all, I think the best reference would be to get the Amiga Developer CD version 2.1 from wherever you can find it. It has rather complete documentation of how to program the Amiga in C and Assembly. The best C compiler for the classic Amiga is currently
VBCC which is a free download. Unfortunately to get it to work you'll need the Amiga headers which normally come on the Developer CD. If you are going to program without it you can download the AmigaOS 3.9 Native Developer Kit
here.
There are example source codes in the
Aminet archives as well as the LZX decompressor you'll need to unpack the AmigaOS 3.9 Native Developer Kit (linked above). You'll also find many other programs for the Amiga there.
For help programming I'd recommend looking on
UtilityBase but be aware that many of the posts there are for the PowerPC-based AmigaOS 4.0 that is currently in prerelease.
Welcome to Amiga.org and feel free to post about your game development questions in the appropriate forum here as well!