There are plenty of tools out there that will help you find the parts you need to build your own desktop computer at home. Newegg is hoping to rope in a bunch of new first-time builders with its own tool. The website is live now, and you don't need to buy the parts (or even log in) to use it.
Newegg is an online storefront dedicated primarily to PC components and electronics, though it's been branching out in recent years. As such, the PC Builder tool is a bit "lowest common denominator," and naturally, its items are limited to the ones sold on the store. So it has a lot fewer options than the de facto standard online PC building tool, PC Part Picker.
That being said, it's not a bad place to start if you're new to PC building. Newegg has three example builds, "Budget," "Mainstream," and "Enthusiast," currently sitting at around $900, $1400, and $2500 respectively. (That's a little pricey for "budget" and "mid-range" builds, but hey, they're trying to sell parts.) Each one can be customized with Newegg's search tools, and if you're ready to pull the trigger, ordered as separate, ready-to-install parts.
Alternately, customers can start from scratch, going through nine different component categories and selecting each one, starting with a processor and motherboard. As you expand your build, the tool automatically contracts the available parts based on compatibility with the components you've already selected.
Note that this isn't like customizing a pre-built PC from, say, Dell. Newegg's only selling you the parts with the tacit assurance that they're compatible with one another---you still have to build the PC, install the operating system, and set up your various programs and settings yourself. If you're unsure of how to do that, might we suggest our sister site?