There are a number of cards available for the DS Lite (proper name for them is Slot1 Flash Cards) most require you to purchase a Micro SD Card, this is what you must load the applicable firmware (operating system so to speak) onto depending on manufacture of the card. Then you can install music, films and other things like Homebrew games etc, cannot go into full detail of the cards capabilities due to them being illegal.
I have used most of the cards available, purchasing many for family and friends (why I often have some sitting at home) If you’re still unsure what they are, drop me a PM.