The kettlebell weight you start with will naturally be dependant on your previous exercise history and natural strength and health levels.
If you have no history of exercise whatsover then you must begin with caution, I would recommend trying a kettlebell before you buy it if it is possible.
If you are unable to try a kettlebell before buying it is recommended that females us a kettlebell around 6-8kg initially and males a 16kg unless you are moderately strong then a 20-24kg kettlebell may be used.
Remember that high repetition kettlebell training will have a great effect on your fitness levels and also that many kettlebells have a relatively thick handle making 16kg feel much heavier than a dumbbell of the same weight.
What Kettlebell Weight Should I use (video)
After selecting the right weight kettlebell for your abilities you’re going to know which kettlebell exercises are most effective