Alright Silent easiest way to do this is Here
Not to be rude, that's not my intentions. You want to make sure your case isn't fitted to the PSU it has now because aftermarket PSU's will not fit in.
Easy example is look at a hp/dell/compaq somewhere and see how all the screws are different for each system. You need a PSU that fits in there.
So you want to find a computer if you don't plan to build that doesn't require any upgrades right away. But best bet is open it up and look around and learn what it looks like and go to youtube and you can find videos of people modding their computers and giving advice.
Also everyone in here is willing to help you with what you need to the best of their abilities.