If you go without a build list:
1. step: Decide whether you want a dedicated Graphics Card or if an Integrated Graphics Processor (IGP) (built in the CPU) is enough. An IGP should be sufficient for osu! and games like League of Legends, but for any somewhat elaborate 3D games you'd want a dedicated Graphics Card.
2. step: Decide on AMD or Intel for your CPU. If you go with an IGP, you are limited to Intel, since as far as I know AMD doesn't build IGPs into its Desktop processors. Intel generally has higher single core performance, which is important for osu, while AMD generally gives you more for less money. (I personally try to avoid Intel because of Meltdown, Spectre, SpectreNG etc. which doesn't affect AMD as strongly.)
For a midrange desktop pc you probably wanna go with either Ryzen 5 2600 or Intel i5 9400 or something of that caliber.
Edit: AMD actually introduced Desktop CPU with IGP with its 2000-series. For example the Ryzen 5 2400G. You should really consider this, as it's quite cheap and it is said, that the IGPs by AMD are faster than those by Intel.
3. RAM: 8GB is minimum, 16GB standard 32 GB luxus, your PC doesn't get slower with less RAM until you don't have enough. Then it's going to slow down enough, that it'll drive you to suicide. So depending on what you do, you might be fine with 8GB for a few years, 16GB gives you some buffer.
RAM is where you have to be careful with technical details. Desktop PCs today typically use DDR4 DIMM RAM without ECC.
A major price factor for RAM is memory clock. CPUs typically have an upper limit regarding memory clock and I don't have any experience going over that. Especially with AMD you have to watch out with the maximum supported memory speed depending on the number of modules and them being Dual or Single Ranked. People generally don't really feel the difference between high and low memory clocks (high being better). 2133 to 2666Hz seems to be standard range for DDR4 RAM.
i5 9400 supports up to 2666Hz, Ryzen 5 2600 2400Hz if you equip the typical 4 RAM slots with 2 Double Rank modules.
Note to Double vs Single Rank: Double Rank is faster for same memory clock (marginally), but Ryzen support for higher memory clocks goes down, if you choose Double Rank.
4. Mother board: The CPU socket must fit the CPU you are buying. You should also watch out, that mother board chipset and processor fit together (look at the processor to see supported chipsets). Other than that (if you aren't overclocking) it's mostly about interfaces: how many RAM slots, how many USB slots, how many PCIe slots with how many lanes. Also important: the formfactor. Standard is ATX, you may go bigger or smaller, but it has to fit your Computer Case. Watch out that if you choose DIMM RAM you don't accidentally pick a mother board with SO-DIMM RAM slots. Or rather pick your RAM, so that it fits your mother board.
5. Dedicated Graphics Card (only if you didn't pick IGP): You can go with anything from 150$ to over 1000$. You can either choose AMD or NVIDIA, although AMD isn't really an option for over 500$. Really depends on what you wanna do, this will be the main deciding factor for what 3D games you can play (things like Assassins Creed) and also how expensive your PC will be. As I said, IGP should be enough for osu, so you can save some money here (however it could get to be a limiting factor on how many fps you can get, but I did never try that). Watch out that the graphics card fits into one of the provided PCIe slots by your mainboard, PCIe versions match (typically 3.0 as of now) and the card fit physically into your case.
6. Power Supply Unit (PSU): Use some online calculator (search PSU calculator) to get an estimate, for how much power your PC will need. Keep some buffer, but not to much, since it will only make the PSU unnecessarily more expensive and also the overall power usage by your PC higher (however if you take a too weak PSU your PC will randomly turn off). Everything else is more money->higher quality. bequiet! is a good brand regarding quality.
7. CPU cooler (only necessary if the CPU doesn't come with it's own): Consider this, even if your CPU comes with its own, since they are generally a lot quieter. It has to fit in your case (read the dimensions of your cooler and case) and also has to support your CPU socket. Also: In general bigger coolers do more cooling while being more quiet and also costing less. So I don't see a reason for not choosing a dual tower cooler, if it fit's in your case. I heard alpenföhn is a nice brand for high value low money products (no personal experience though).
8. Case: Read outer dimensions to find out, whether it fits in your room and inner dimensions to find out, if it fits your cooler and graphics card. Look for which mother board form factors it supports. Also look for possibilities to put in hard drives. Rest is build quality and nice features like build in fans, LED blink blink or hard drive mounts in a way, that reduce their noise.
9. Storage: If you don't have it already, you'll need at least one storage device. Classic hard drives give more capacity for less money but are fucking slow (if you are used to SSDs). So I really recommend choosing a SSD at least for your Operating System and main programs (and save money with other components if necessary). Windows + programs typically take up 100GB. Although you can try to keep it under 50GB it grows to that size pretty fast. So the SSD should be at least 125GB. You can choose to buy a bigger SSD for your personal data (music, pictures, etc.) but it's cheaper using a hard drive for those (you don't need SSD speed for your music). However using multiple storage devices in your PC can make data management slightly more complicated.
10. OS: I strongly advice you to stay away from Windows 7. It's only supported until 14th January 2020 and after that you shouldn't use it in the same way you shouldn't use WinXP now. Also it's really old and just installing security updates for the first time will take multiple days if you don't know the shortcuts (with shortcuts it's going to be a few hours).
Other than that you have the choice:
Win 10 Home: Do like being a beta tester? Because with Home you can't really delay feature upgrades, until Microsoft fixed the obvious and fatal bugs.
Win 10 Pro: Expensive but good otherwise. You still have things about privacy or other things people typically don't like about Windows 10.
Linux: Either be an expert and have some time on your hands, or be a beginner and have a lot of time on your hands.
Apple: Not an option for self built PCs.
Finished!
Now spend some weeks, pondering whether you really picked the right parts. Then order, wait impatiently a few weeks until all parts arrived, start building your PC and realize you forgot some little thing, you have to order wait another week for.
But when you turn on your PC for the first time, you will realize, that it's a hundred times better, than any pre-built PC you could have purchased.