You have left out the most importanmt information, your motherboard make and model number.
Whatever motherboard your have (or are getting) will define almost everything else.
The documentation that comes with the motherboard will tell you what CPUs it can take, what sort of memory it will take and how much it will take, what graphics card "slot" it has and so on.
The motherbord is the heart of the system and you start with that and build everything round it (or you buy the CPU you want first and then find which motherboards it fits in).