Hello,
I recommend you to start with the 3d/meshFromCamera,
graphics/polylineBlobsExample and addons/opencvExample, from the examples folder.
If you really want to play around with openCV in OF, you should check Kyle Mcdonalds' ofxCv. It's more up-to-date and with much more examples. Also, you can check the addons website to get more approaches.
Have fun!