Anyways, you need a decent computer. I'll leave that for you to discover on your own.
Things you need:
1. Download No$GBA.
2. Get Pokemon Diamond/Pearl. I suggest Bit Torrent. I'll give a direct link sooner or later if enough n00bs can't find it.
Extract No$GBA, and the ROM to their respective folders, run the No$GBA.exe and load up the ROM. Have fun playing, and be sure to configure your controller options. If you have any problems be sure to post here.
The game usually runs at 90% speed for me, but when I configure the performance tweak, it should run at 100%. I'll post them here when I figure them out.