Best way, is to use Kies to connect your phone to your PC, then Move / Delete some of the music / photo files from your phone.
What counts to run programs is the memory space available for it to use in your internal memory. So you need to move apps to SD card to free-up some space in your internal memory. OK, now the main problem that you are facing is a memory shortage. Apps you install on your phone, are first installed in its internal memory. Also your system uses this memory to store needed info like SMS`s,MMS and other stuff needed to operate. After a while your memory becomes almost full, so programs running find almost enough space to use