Welcome to the site! 15 months is such a fun age. I miss having little ones!
My husband and I started the idea by attributing a point value to certain behaviours or chores we wanted our kids to exhibit. For instance, if they used their words instead of tears/yelling to get their point across they would get 2 points for the day. (of course this was when they were much younger). I also questioned them about things they would like, be it extra TV time, an outing, or a toy. I would give a point value to that "prize" and they would work toward earning the prize by showing good behaviours/doing chores, and accumulating points. We always chose a range of prizes having very few to very large point values. This way the girls could either bank their points to use toward bigger rewards, or spend them as often as they liked for little things like a trip to the ice cream or candy store. Prizes also don't have to cost anything. They can be free such as an extra hour of TV time a day, but when we started this out we wanted our kids to learn how hard it was to work for material things. This was in response to the constant "I want, I want" at the store.
I'm not sure if I clarified this enough for you but if you have any questions feel free to post here or email me at susan@childzilla.com.
Susan