Welcome to Amiga.org!
I don't know where you can get Amos I don't think it's available any more. It's not very good anyway you should learn assembler and use AsmPro or Devpac - it's much more powerful but it will take you a little longer to learn than Amos.
There is a good guide for beginners to assembler
here. It is by Jolyon Ralph, who is quite a famous Amiga programmer.
