How to start implementing your own computer?
If you really want to do this, I suggest looking at old processors; something of the 6502 or 8080 vintage. In those days, you could take a cpu and some external chips and assemble a working - if limited - computer. Newer processors are much more exacting and challenging to work with.

Note that this is huge undertaking; do a search for the apple 1 schematic, and you'll get some idea of what it will take. Then you get to start writing software for it, and you'll get to do at least some of it in assembly. 

If you're actually trying to get things done, the current crop of microcontrollers are so much easier to use; you can write in C/C++, they have great built-in capabilities, and they're cheap.