step by step to build my own robot

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ROS is probably not a good solution to your problem, as it is not designed to help you build a robot, it will however help you use one, provided that you have or add support for ROS to it.

There are tons of blogs and resources all across the internet that talk about making robots, from the simple wheeled line follower to the full-fledged humanoid robot. Google how to make a robot and pick your choice.

A few pieces of advice:

• if you want to use ROS on it, I suggest you go for an x86 architecture, with support for Ubuntu Linux, as it will provide you with optimum ROS compatibility
• you can start with simulation only, so that you get familiar with algorithms and methods, if not already done, I suggest you follow the Gazebo tutorials on how to make a small robot like this one, how to write plugins, and ROS nodes
• please try to break down your problem before submitting it to the community. If you have a specific issue with a piece of software, a sensor or actuator, then you are likely to receive an answer if one exists. Questions such as how to build a robot are way less likely to receive an answer

Short answer: google is your friend, ROS answers is not really the good place for your question