Back in the early days of building your own computer, ages and ages ago, I was there and did that. Fast forwarding decades to today, I have some tips on getting parts and figuring out what you need. These days, I personally work with servers, and advanced networking for companies and businesses, but there is still fun in putting together a workstation at home and here are some great places to get stuff you need.
Here is a great place to start online. PCPARTPICKER. They have a great system builder that you can pick parts and put together something compatible. Checkout the COMPLETED BUILDS section as well for some assembled machines on your budget.
The greatest part of this is the parts list you can search anywhere for the best price, but in most cases they show you the best price currently advertised. You might want to get everything at a single place, but you can weigh that option when you get the design together you want.
Now there are a lot of places you can get computer parts, so it never hurts to check around and maybe save some here and there.
For those that are upgrading, maybe you already know the part you want. Check out a site like CAMELCAMELCAMEL for that Samsung hard drive you had an eye on. The price has bottomed out on it, but at least you know you are not at the top of that scale.
There are a thousand videos on assembling a computer, so not going over that here, but just wanted to share some of the sites in my toolbox that I use when planning.
Hope this helps!