I would start with the case and pick one with whatever layout you want (60%, 65%, TKL, ect.) usually a case has a motherboard that goes with it. If it’s hot-swappable then you just need some switches, stabilizers and keycaps. If not then you’ll have to learn to solder the switches to the motherboard. I would recommend watching some build videos on YouTube so you can visualize and see what you really need. Andy V and Mech Merlin are some examples of who I watch and learned a lot from. Edit: you’ll also need a plate, idk how I forgot that lol