This is a very very broad question because there is so many different types of "Robots". There is really no one site for all robotics, you need to narrow down your search to your desired type of robot.
If you already have some hardware you should search for forums related to that hardware.
As I already said robots come in all shapes and sizes, from products designed for young children to products designed for the Terminator. From stationary beasts to tracked, wheeled, airborne, or submersible. So you need to decide what type of robot you're interested in playing with, then research, research, plan, purchase and have fun.
then, what kind of robot that people used to learn at first time?
I started with BEAM robotics and I have nothing but praise for Lego Mind-storms if you have the spare change for it.
i wanna ask, can anyone suggest good C++ robotic forums?
To address your question, both Arduino and Raspberry Pi support C++ for programming. Arduino uses a C++ like API and the Rasberry Pi is built on a Linux kernel.