What's that Star Wars song called that was playing when the main character entered a bar in the first movie?
“Cantina Song” or alternative known as “Mad about me” was performed by the fictional band Figrin D'an and the Modal Nodes. The song was really created by John Williams, who was given direction from George Lucas who reportedly said: Can you imagine several creatures in a future century finding some 1930s Benny Goodman swing band music in a time capsule or under a rock someplace — and how they might attempt to interpret it? 

It was played it in Chalmun's Spaceport Cantina when Luke Skywalker and Obi-Wan Kenobi entered to find passage from Tatooine to Alderaan in Star Wars: Episode IV A New Hope.
Is this music hard to play on a piano?
The difficulty of playing Cantina Song on the piano will depend on the individual's piano playing skill and experience. If you are a beginner pianist, you may find the song challenging due to its fast-paced and syncopated rhythm. However, if you have some experience with playing the piano, you may be able to play the basic melody relatively easily.

It's worth noting that John Williams is known for writing complex and challenging music, so even the simplest-sounding pieces from Star Wars can be difficult to play accurately. If you're interested in playing Cantina Song on the piano, I would recommend starting with a simplified arrangement and working your way up to a more complex version as you improve your skills. There are many sheet music books and online resources available that can help you get started.