Depends on your budget, but there are several things to consider when building a gaming rig.
I usually pick a CPU and build around that, because people tend to make a lot of rookie mistakes like buying an Intel CPU and an AMD motherboard and such.
I personally go with AMD CPUs because their price point is a little better, with comparable performance to the Intels.
These days, nothing below a quad core is going to cut it. A good quad core AMD will run you about 100 bucks. Pay attention to the CPU socket type, because you will have to get a motherboard with the same socket or else it won't work. Another thing to pay attention to is compatible RAM speeds on the motherboard you select.
These days, 8GB RAM is the absolute minimum. 16GB is preferable.
Most people like to do Solid state drives nowadays, and they work great but they're still a wee bit pricey and configuring an SSD/HDD dual set up is not something I'd recommend a beginner builder attempt. A good 2TB hard drive with a 64MB cache should work well.
Now, video cards are very divisive among people because you have your hardcore AMD and Nvidia people. In my own personal rig, I'm using an AMD R9 270x 4GB card, and it works great. Maxes out Dying Light and the card itself was only $220.
One thing to look out for when buying cards is to check if they have any 6 or 8 pin power plugs. Most higher end cards have 2 8-pin plugs, so you'll need a good power supply with two of these plugs or else your card won't work. A decent 600W+ supply should have everything you need.
Lastly, you'll need a case with good airflow and is large enough to accommodate the video card, as most higher end ones can be 10+ inches long. Just read reviews of cases to find out a good one for you.
The Witcher 3 is gonna need a pretty decent rig to run it on higher settings, so I'd be ready to spend about 500-600 bucks on a good build.