Basically put everything else besides apps on the SD card. Mostly photos, videos, and musics like Snipergod said, or other stuff such as documents or things you don't need. Don't install apps on the SD card, plus I don't even think lots of phones can do that anyway (with the exception of rooted phones). You might want to get at least a class 10 SD card, they aren't that pricey, and you get performance.
Let's put it this way: I wish I had put my videos and pictures on my SD Card when i stepped out of the pool and my phone was in my shorts. Something felt funny and heavy in my pocket. An SD Card can still be read after it's been in water. A phone cannot.