Which weight to start with for a 90kg guy with little experience and poor shape?
I think most people are delusional though to believe they are going to stick with home kettlebell workouts if they don't like the gym. Most likely the kettlebell will be an expensive door stopper in a few months.
I have just met so few in shape people in my life that don't go to a gym. I have a pretty decent home gym and a good assortment of kettlebells but the external gym I go to just has so much more.
The kettlebell reddit (/r/kettlebell) is a very good place to learn from - lots of people helping each other get the technique right. Seems the most common mistake is "hinging too early" in the swing.