When you're building a new PC, you want to buy the best parts for your specific use of it, right? You don't want to pay too much for something that's overkill, but you also don't want to pay for something that just doesn't get the job done. Have no fear, I will show you what you need to buy for whatever use you may have for it.
CPU
Central Processing Unit. The brain of the computer. This part is the most important in determining the general speed of your computer.
Intel i3 CPU | $100 to $150 - This is mostly for those who want to have a good, reliable CPU for daily use (such as browsing, watching videos, maybe the odd online game). It's not recommended for gaming, but you can run some low-end ones on this CPU if you have a suitable GPU (we'll come to GPUs later).
Intel i5 CPU | $150 to $250 - Most of these CPUs are very powerful and blazing fast for those most uses. They are more expensive than the i3s, but their worth it. If you have the cash for this, buy it. This CPU will also work well with gaming, though you might not be able to max out any high-end games without a good GPU.
Intel i7 CPU | $300 to $1 000 - And now we get to the best of them all. The i7 series is there for the superusers: those who want to do heavy gaming or multitask like crazy. Their very expensive, so I wouldn't recommend this to most average users, but if you are willing to dish out a few hundred bucks for a good one, go ahead.
GPU
Graphics Processing Unit. Ranging in cost from as little as $20 to over $1 000, these are the beasts which run those high-end games at ultra graphics. However, your $20 GPU won't even be able to run those games at more than low settings. If you look deep, you will find some GPUs that are good enough for gaming but won't break the bank.
GeForce GTX 750 Ti | $80 to $130 - The cheapest GPU which can perform well enough for decent gaming, the GTX 750 has been called the workhorse of GPUs. Even though it is a bit outdated and doesn't sport that much memory, it's good enough for your new gamer.
Radeon RX 480 | $225 to $275 - The best budget graphics card out there, the RX 480 8 GB version performs well above its price range. Arguably better than the GeForce GTX 970 below, this GPU works wonders. My preferred choice for budget gaming.
GeForce GTX 970 | $250 to $350 - A good solid graphics card for your average or casual gamer. Although it doesn't have that much memory (only 4GB for the more expensive version), it is good enough to run most games with medium settings at 60 FPS.
GeForce GTX 1070 | $350 to $400 - If you're willing to pay a bit more (about $50), you can get this beast. With 8 GB memory and a high core clock, this GPU is sufficient for most gamers. It can run any game at medium to high graphics settings.
RAM
Random Access Memory is what you need to multitask. If it wasn't there, where would your other tabs of your browser or your second window of Photoshop be? They all have to be stored in the RAM, where they can be quickly accessed.
4 GB RAM | $20 to $30 - Suitable for your average user, 4 GB is good enough to do some multitasking (albeit not too heavy). It's affordable and you could easily buy another stick of it if you need more.
8 GB RAM | $40 to $300 - For the more heavy multitaskers or gamers, 8 GB RAM should be enough for running most games as a relatively high setting and also having a lot of Google Chrome tabs open.
16 GB RAM | $70 to $200 - Only buy this is you're doing heavy gaming or running a ton of monitors with about a hundred Google Chrome tabs open. This is only for the serious superusers.
Storage
This is pretty much self-explanatory. The more space you have, the more you can store. The faster the storage is, the faster you can store it. Now, there are usually two types of storage: Solid-State Drive (SSD) and Hard-Disk Drive (HDD). SSD is much, much faster and means that your PC will start up inside ten seconds (usually) and your applications, including games, will load much faster. However, it's a relatively expensive technology.
500 GB of space | $30 up HDD | $100 up SSD - Enough for your average user who may game a bit and store all his photos and videos on the drive, 500 GB is usually cheap and easy to find. If you're not storing anything that takes up a ton of space, go for this.
1 TB of space | $40 up HDD | $250 up SSD - More for your gamer who downloads a few 50 GB games onto the drive, this can also be for those who store a lot of pictures or videos (for whatever reason). Great for backups, too.
2 TB of space | $50 up HDD | $600 up SSD - Really, I don't think anyone needs this much. However, if you do want 2 TB, don't buy an SSD unless you've got an endless supply of money, since you can easily buy another brand new PC for $600.
Other Parts
Power Supply Unit (PSU) | $20 up - Make sure your power supply meets your needs. If your graphics card takes up 200 Watt, your CPU another hundred or so, and your RAM also about a hundred, but your PSU is only 350 W, then you'll be in trouble. Go for a 500 W up one. They're not much more expensive than worse PSUs.
Motherboard | $40 up - I cannot stress this enough: make sure all your parts are compatible with your motherboard. The motherboard is a board on which most parts sit. If they don't fit on it, then how will your computer even work? Do your research before buying one.
CPU Cooler | $10 up - I would recommend a CPU cooler to anyone, even someone who really isn't pushing their PC to its limits. It keeps your CPU cool, making it last longer and perform better. And hey, it's cheap.
Case | $20 up - You've got to have a computer box which holds all the parts, right? Don't go for a really expensive one unless you've really got a lot of cash to spend. Go for a nice looking one which has all the ports you need.