Level
It’s almost impossible to play a song correct the first time you see the score. It's best to start by trying different parts of the song. You must first understand the meaning of the notes before you can really play a song.
This is often best done by paying close attention to the slurs. Make sure that you can easily access all keys within one slur. After a few times you usually can play that group of notes smoothly, so that it sounds as it should sound. Then start working on the next slur, just like the first. With each new slur you start from scratch, as if it were the beginning of the song.
When you can play all the individual groups, you can 'glue' these groups to each other. Then the song will automatically begin to sound better and better, in the right rhythm. And if you still can't figure it out, then try singing the words to the song. In your head (or out loud). And listen to the clip while you read along with the notes. In the song One thing van One Direction you can see how slurs make the song easier to read. And easier to practice!
Another useful tip: it is better to practice a few times shortly, rather than once for a very long time.