You can start building robots by buying some ready made stater kits like LEGO etc.
Of if you want to go deep into the subject you can buy electronic stater kits and Arduino microcontrollers from eBay.
You can also buy various types of sensors from eBay too
i hope this helped you ☺
There r different types of robots .u can make a action model from cardboard and paper.i have bought a plastic model which we can join  and make a solar panneled robot from annies present in stationary stores.making the two gears r very important.take care that they dont get broken..u can also buy batteries otherwise u buy a lego.
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