There are a few different editions of sheet music for Hedwig's Theme, based on musical ability, so I have found a few good ones for you to try:
- The first is from Musescore which allows you to not only read the sheet music in time with it being played, but also to listen how it should sound. This edition is of medium difficulty:
http://musescore.com/user/14478/scores/25026 - Jellynote do much the same as Musescore, but the difference is that this sheet music is more difficult and probably closest to the original John Williams piece:
https://www.jellynote.com/en/sheet-music-tabs/john-williams/hedwigs-theme/50fd71d60be58724624f9f76#tabs:%23score_P1 - The third version is a simplified piece which may be easier to work with if you are a beginner and has been kept to two pages. It is in PDF format so can be downloaded onto your phone or tablet easily:
http://teacherpress.ocps.net/melissasandusky/files/2013/11/Hedwigs-Theme-sheet-music.pdf
Finally, here is a video from the programme Synthesia which shows exactly where the notes are played on the keyboard. There are lots of these tutorials on Youtube and can be a good resource for visual learners: