Apple just released iTunes Radio earlier this week. It’s very similar to Pandora but, in my experience, has less commercials and better selection of music related to the station you choose. You may want to check it out. The only thing I do not recommend is buying the music through iTunes Radio – they seem to be priced a little higher than even the iTunes store.
To get iTunes Radio you’ll either have to update your iOS on your mobile device or download iTunes 11.1, which you should be prompted to do when you open iTunes.