Human: I would like to build a computer. What steps should I take to make it perfect? How do i find the best parts for the budged and how do I know if the parts fit together?
Assistant: The first step that you should take is to think about what you want to use the computer for. Are you going to use it for office work, for programming, for gaming, or something else? Afterwards you should determine which aspects of the experience you care about. As an example, you could think about how important noise levels are to you relative to the size of your monitor. 

Generally you will need at least the following parts to build a functional PC: a motherboard, a central processing unit (CPU), random access memory (RAM), a solid state drive (SSD), a power supply (PSU), and a PC case. You will also need peripherals that allow you to interact with your computer, in particular a mouse, a keyboard, a monitor, and either speakers or headphones (but your monitor may have built-in speakers). For a gaming PC or a PC that is to be used for certain computation tasks like training or running neural networks you will also need a discreet graphics processing unit (GPU). There are many online tools that can help you with picking specific parts, for example:

https://pcpartpicker.com/

The above website will also automatically tell you when parts are incompatible (mostly between motherboard, CPU, and RAM or because the case is too small). However, ultimately the "perfect" choice of parts is subjective and due to the constant influx of new and better parts the answer to which parts are "best" constantly changes. The best advice that I can give you is to seek out a human with knowledge of PC building in general and that also has up-to-date knowledge of which parts are on the market right now. This human can either be someone that you know personally or a dedicated online community that is about building PCs. Regardless of whether you seek more advice or try building a PC on your own I can still give you the following tips:

* The CPU and the GPU are frequently the most expensive parts of your computer. Look up benchmarks and reviews for them since a large part of your budget will be spent here.
* For the same budget you may be able to get better parts by buying used parts rather than new parts.
* Monitors have many different aspects that you may want to consider. Different technologies like TN, VA, and IPS have different pros and cons. For gaming you should also consider buying a monitor with a high refresh rate since this is frequently reported as providing a better experience compared to the conventional 60 Hz; at high refresh rates the CPU becomes more important relative to the GPU.
* How will you connect your PC to the internet? The use of an Ethernet cable is recommended but if you want to use a wireless connection you will likely want to buy a motherboard that supports it.
* If possible, buy at least two sticks of RAM rather than a single stick of RAM. Consumer PCs can read from two sticks of RAM simultaneously so this gives you better performance. Make sure to put the RAM into the correct slots.
* Do not buy the cheapest power supply that you can find! A poorly made power supply is a fire hazard and may cost you much more than the $10 that you "saved".
* There are soft factors that may still play a large role in how much you enjoy using your PC: how much noise does it make, how comfortable is your keyboard to type on, how comfortable is your chair to sit on, etc.
* For noise levels in particular an aftermarket CPU cooler as well as a high-quality case and case fans are important. According to tests by GamersNexus, airflow cases are objectively superior to cases that are more closed-off in terms of how much noise they need to make to cool the components.
* You will also need to install an operating system on your PC, typically either Windows or Linux. You can buy cheap OEM Windows licenses on the used market but make sure to check the legality of this practice in your legislation beforehand!
* You do not strictly need an SSD but installing your operating system on a hard drive disk (HDD) will greatly degrade the performance of your PC so it is not recommended. Still, you may choose to buy additional SSDs or HDDs to increase the storage capacity of your PC.
Human: Where can I buy parts from in the US?
Assistant: There are many retailers that sell computer parts in the US, both online and in brick-and-mortar stores. Here are a few popular options:

Amazon: Amazon offers a wide selection of computer components and often has competitive prices.

Newegg: Newegg is a popular online retailer that specializes in computer hardware and software.

Micro Center: Micro Center is a brick-and-mortar retailer with several locations in the US that offers a wide selection of computer components at competitive prices.

Best Buy: Best Buy is a popular electronics retailer that offers a variety of computer components, including CPUs, GPUs, motherboards, and more.

B&H: B&H is an online retailer that specializes in a variety of electronics, including computer components.

Fry's Electronics: Fry's Electronics is a brick-and-mortar retailer with several locations in the US that offers a wide selection of computer components.

It's always a good idea to compare prices and read reviews from other customers before making a purchase to ensure that you are getting a good deal and a quality product.