Opening track

I think that is where research of your clientele or the vibe of the party will be. I’m sure the birthday boy or girl has a playlist. And because it’s a friend, maybe the taste in music is similar which in that case shouldn’t be difficult or a big deal what the first song is. Are you starting the music initially or when are expected to start dancing. If it is just background music then I would suggest something that is a happy median between all ages. Of course this is months after you posted this question so I imagine you have answered this question for yourself several times over.

The number one thing to think about is the atmosphere you want to create with the music, and how you want the crowd to flow. The opening track is very important for setting the atmosphere. Is it more of a dance party or just a more casual environment where you will be playing background music? If you are doing a dance party, I’d recommend Clarity by Zedd feat. Foxes, or something of the like. Everyone knows it, it is danceable, and people can sing along to it, especially the younger crowd. If it is background music I would recommend any sort of pop song that isn’t too high in terms of energy level that everyone knows. You can raise things up as you go along. I would stay away from anything with profanity.