how can make RGB signals with 8051 ????
First, do you understand how a TV pattern generator works?
If not, that is the first thing you need to study - completely independently of the 8051 or any other kind of microcontroller.
Once you understand how a TV pattern generator works - ie, what signals it needs to generate - then you can start to think about how you might implement that on a microcontroller in a general sense.
Then, when you have a general idea of what needs to be done, you can start to think about the specifics of how that might be achieved with an 8051.
See: www.8052.com/.../160143
See also: www.8052.com/.../
See also: www.8052.com/.../23781
It seems this has now moved to: video-demystified.jackfamily.net/.../