Welcome to Ponyville! ... I mean, welcome to Everypony Sings, where fan Neonz has shared some great resources for Pinkie Pie's Welcome Song.
The Welcome Song is short but sweet. Neonz shared a whole bunch of resources for this fun song. If sheet music is what you want, then you can check out pdf sheet music with and without lyrics. There's also .mus files with and without lyrics. Then there's a midi. Finally, you can listen to the song in the video.