i am going to buy iphone12 but i want to know if i can still use old cablesof my iphone7 ?

Yes you can, since the iPhone 12 has a lightning port, but using the iPhone 7's charger might be a longer process.

You can buy a more powerful charger (i.e. this one https://www.apple.com/shop/product/MHJA3AM/A/20w-usb-c-power-adapter , and use the cable that is included with your new 12.

Or, buy a MagSafe wireless charger: MagSafe Charger - Apple

Regards

Giulio

Yes.