As well as the Tutorials here, I suggest you go off to the Keil site and request their free CD:
http://www.keil.com/demo/cdrom.htm
This includes demo versions of all their products, and a wealth of 8051 support info.

Install uVision & C51, read the uVision Getting Started guide, and work through the example Projects.

As a C++ programmer, you will also need to read the C51 manual to find the 8051-specific features. There is even a section on writing optimal 'C' for the 8051.

As a freebie, of course, the demo has certain limitations; be sure to read http://www.keil.com/demo/limits.htm