People often use PCPartPicker since the GUI is intuitive and straightforward, you can also find what needs to be found using the categories/options on the navigation pane to the left of each parts selection.
PC Part Picker is the one used most often on here.
As for 'which parts will be best' - that comes down to what you want from the system. What software do you plan on running (as from the above it sounds like games are the secondary use)? In terms of games what kinds of games are you wanting to play, and are you after highest quality visuals or more concerned about game performance? What kind of screen will you be using (most importantly, what resolution and refresh rate?
Final question, what kind of budget are you looking at?
There are lots of options components wise so helps to understand what you want from the machine.