And the Roms for it on this page. It has Pokemon crystal plus about 8 others.
Once you download the Roms you want open up the emulator. Once you open it up click file open then open the Rom you want to play. Before the Rom or the emulator work it may ask you to extract the files, but then it will be fine. I'm not great with computers so it shouldn't be hard to figure out. I hope it works!