I am trying to build a desktop just use for retro gaming, could someone help me please I just would like to know what I need to get for the best time
Emulation is mostly a single-threaded affair, so you want the best single-threaded performance you can get. This used to mean Intel was the only option, but the Ryzen CPUs from AMD are solid performers, as well.
GPUs are mostly needed for shaders, so you don’t have to spend too much there. Any midrange GPU should be able to handle most of the shaders well enough.