i need info on replacing brake pads/rotors, (base, not sport)

Zeetah

ive been hearing screechings sound as ive been braking the last few weeks or so, whats the best way to tell whether its coming from the front or rear, and whether its the brake pads or rotors that need replacing, i dont have the cash to upgrade to bbk's atm, so i just want to get decent pads and or slotted/cross drilled rotors, which ever of the 2 that is needed, hopefully just pads, since im assuming those are cheaper, and if you know where to buy pads or rotors from the gta, in a store preferably where i can pick it up would be helpful, or even online if thats my only option, thanks