Find out what music the guy you are trying to impress likes. Consider learning a piece that is in his preferred style; for example, learn the theme song from "The Office" if it is his favorite television show. You may have to learn a song you do not like.
Print out or buy sheet music for the song. Alternatively, you might be able to learn a very simple tune by ear, but you will likely need some music.
Play through the piece a couple times and make notes or highlight any sections that are difficult. Practice these more challenging parts more often than the rest to get them up to the same level of proficiency.
Listen to a recording of the song if one is available. Make note of any nuances in the music, such as dynamic (volume) changes or rubato (slight slowing or quickening of the tempo). If possible, watch a video of someone else performing the song. Watching and listening to other performers will help give you ideas for your performance.
Tape yourself playing the piece after you have practiced it. Practice the piece again and fix any errors. Practice the piece as if you are performing it for the person you want to impress. Ask friends or family to be an audience and give you tips on how to improve your performance. Be emotive during your performance because it looks and feels impressive.
Perform the piece for the person you want to impress. If you perform like you practiced it, chances are you will be successful.