This is again a common question I come across. I first thought about it when discussing my gaming CPU first, so go read that if you’re in need of a buying guide.
I want to make this as brief as possible so it’s easier for you to just get to where you need. So to actually play Pokemon on your computer, you’re going to need an emulator software. Since Pokemon was on multiple systems (GameBoy Advance, Nintendo DS, etc.) you’ll need to know which system you want to emulate. Let’s pick the GameBoy Advance, for example. You’ll need a GBA emulator. There are tons out there online, but I personally prefer EmuParadise’s emulators. They always work and are relatively lightweight. Here’s the link to the GBA emulators I’ve been using for a few years. CoolROM.com is another great one.
Now you have your emulator. Just like on a real GBA, you’ll need the actual game. On emulators, this is called a ROM. You have to download each game you want to play. Since we want to play a Pokemon game, go ahead and search for a GBA Pokemon ROM. After you figure out how to use the GBA emulator, which should be pretty simple, you can locate the ROM file and play it directly. Each emulator has different control schemes since you’re using a keyboard now, instead of the actual GameBoy. Figure that out and you should be pristine.