Yes, more reps are best for losing weight/toning up - don't worry too much about what size weights you use as it'll take a LOT for you to build huge muscles! A good guide is that if you're doing 15 reps, choose a weight that means that you really struggle with the 12 to 15 repetitions of each set.
You haven't said what gym you go to, but most gym instructors will be happy to give you some pointers if you ask for help!