if Raghu has convinced you to do the right thing (drop BASIC) there is the industry standard assembler free from www.metalink.com. To dive into assembler or C you will need to know the contents (you can not in one go know the whole thing, but you need to get the 'picture' and an idea what info is there) of the first 3 chapters of the "bible" <--- links has the link.
If you plan eventually to start using C, it will be a better ides to use the assembler in the free demo versions of the Keil or Raisonance software (plenty of links)

Erik