You don't have to build a PC to fully experience PC gaming
First of all, don't ever buy a "gaming" desktop. EVER. It's either going to be tremendously expensive (Alienware) or severely underpowered for what you pay (CyberPowerPC, iBuyPower, iMac). Most of the time, they toss in a along with some bad $50 card and your games run terribly. Second of all, you can put your newly-purchased hardware in any full-size desktop. As long as your PC is relatively decent and you add a ~$100 graphics card to it, you will be able to run modern titles very well. Although, building is still the best choice because it will last much longer and offer better customizability and value.
If you don't want to build one(which is weird, because ), my recommendation to you is to buy a full-size decent non-gaming desktop from a retailer, buy a graphics card, andinsert it into the PC yourself. It's no more difficult than upgrading the hard drive on a console or replacing a home air filter. Once the card is in, just install the drivers for it from the AMD or nVidia website, and your games will be good to go. demonstrating and explaining how cheap and easy it is to turn an 'office box' into a gaming machine.
Lastly, since no one has responded here in ~8 hours and I can't give you much more guidance, you should consider posting at /r/buildapc, people there will probably be more able to help you.
Watching a 20m or less video will show you how to properly build a computer. It's not very hard and all you need to do is plug things in and also install drivers and your OS, which are also just plug and click next kinda things.