make sure your eating enough calories for your body, a minimum of 1500, because your slim you must have a high metabolim so I would say maybe 2000 calories a day.

Don't do much cardio like cycling and running because that will burn fat not build muscle.

Squats are good to build leg muscle, anything that would bulk 'em up a tiny ( but not too much, just enough to give 'em definition)!
hope this helps
xoxo
maggi

__________________