Nikon d70 bracketing

I found the BKT button but I don't know how to set up my camera for bracketing.

Your owner's manual has a section just for that. If you can't find it, you can download it from the Nikon website.

Press and hold done the BKT button, and turn the main control dial to select how many frames you want. Use the sub-command dial to change the exposure value different between those shots.