Hello @jairojosy and welcome to the RobotShop community!
It’s awesome that you are interested in robotics at such a young age 
May I ask why the Raspberry Pi was not enough for you?
I don’t have experience repurposing an old CPU for controlling a robot so I won’t be able to give any advice in that regard, however as you are building a humanoid I thought this topic could be of interest to you:
https://community.robotshop.com/forum/t/lynxmotion-ses-v2-humanoid-robot/
And here are some boards that might interest you:
As you already have a Raspberry Pi you could try adding a USB Stick/Dongle to add some extra processing power (you can find some options in the link above)
I hope that helps!