PLease all bikers help!?

i am making a custom bmx from Dans Comp online and i need help knowing what sizes fit what? like what head set fits what stem which fits what handle bars? and what chains fits what chain wheels/freewheels and what brakes are the best/pads and what seat clamps and grips plus plugs and post and seats please answer this i neeed help , and what pegs will fit on the left side for a fit tech one and how do i know what sizes should i get? please tell me everything or a website where i can learn all this! thanks people it means alot!