Standard advice (assuming Keil tools):

First, you need to read the uVision Getting Started guide, and work through the example projects in it.

You need to read the following documents - commonly referred to as "the bible" for the 8051:

http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_ARCH_1.pdf
http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_PROG_GUIDE_1.pdf
http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_HARDWARE_1.pdf

Here are some other introductory & reference materials:

http://www.keil.com/books/8051books.asp

http://www.8052.com/books.phtml

http://www.8052.com/tutorial.phtml

You will need to read the Data Sheet for your particular processor, and the Manual(s) for any development boards, etc.

You will need to read the Manuals for the C51 Compiler, A51 Assembler, etc, etc,...