How to ground myself while working on computer?

I'm going to be building a computer for the first time, and I do not want to risk damaging the parts, I plan on getting one of the ESD straps but I'm wondering, where do I attatch this to? my case is painted black and the inside is coated white I dont think theres any bare metal, and if i use the power supply method, does it need to be plugged in and switched on at the main? or just simply plugged into the socket, any advice is much appreciated, thanks!