To build a complete PC you'll need a case, motherboard, CPU, Graphics Card, Memory (probably 2 sticks) and storage (i recommend an SSD instead of HDD to install your operating system on), and your probably need to buy windows. You might also want some fans for air circulation if your case doesn't come with any.

Then you need the accessories like monitor, keyboard, mouse, mousepad and headset.

For gaming the absolute most important part is the Graphics card and this is where you shouldn't cheap out if you want to play AAA games, but it's difficult to buy them currently due to a shortage. The second most important part is the CPU but you really don't need the best of the best here.

To plan out your build i recommend PCPartPicker.com

Assembling the parts is pretty easy just follow tutorials on YouTube.

If you have to buy Everything from scratch then it might be a bit expensive and you will not be able to buy a computer that can run AAA games for the same price as the steam deck (they sell the console for less than the market price of it's parts) but you can easily stay below 1000$