There are a lot of free compilers for the PIC Family of microcontrollers for C, Pascal, Basic languages. But if you need to code in PICs ASM then the best resource is the 16f84a's manual from the manufacturer (4th item on the link), then progress through Development Tools Documents and Application Notes sections.
Hi...m just a beginner...could anybody tell me from where i could find a gud tutorial for pic16f84..............
Google "Nigel" there are some very good tutorials along with homberew boards for experimenting. Also log into talkingelectronics.com, they have kits and instructional articles. The web is full of good stuff!