how do i make 8 bit music?

Type MML in the editor and press the help button. (The one that looks like [ ? ] ) It'll show you a basic tutorial on MML, which is the main music-making format for SmileBASIC.

START COMMENT @ "Oscar" PRINT "Thanks for info!" SEND LINE-3 END

its only piano keys?

To use other instruments you need to write @ followed by a GM number in your MML string. You can find all default GM numbers in the SmileTool. To do 8-bit style music you should probably use GM 80 and 81, which are basic square and saw waves.