Levers allow us to play in different keys without retuning the harp. So you can get by without levers for quite sometime if you are just beginning. However if you would like to play in different keys you will need a few levers. That depends on the type of music you play and the level at which you are playing. A full set of levers allows you to play in every key.
Many people choose to add levers over time to help spread out the cost.
For example you can just add levers to the C, F, and B strings. They way you do this is by tuning the string flat and then engaging the lever during normal play to give you the natural note. Then when you want the note flat you simply disengage the lever. For example - a harp with a full set of levers is usually tuned to Eb when all of the levers are disengaged. So the strings would be tuned to C D Eb F G Ab Bb C etc.