If you want the best performance, you should install the OS and a few primary games/apps on the SSD, and use the rest on the HDD. You also have an alternative method using some of you SSD space to cache the HDD with a tid bit called "Smart Response", if you decided to put the OS on the HDD. But i wouldnt suggest it personally.
thewisdomp is right put your games and stuff with usually long load times on the SSD most will use their SSD for their OS and games and put music, movies (videos) and photos on their HDD. Another option is also to use your SSD regularly and use the HDD to simply back up everything to the HDD many like to do this also!