Do I have to be the athletic type?
Not really, at least not to kitesurf casually. Since you should normally use a harness, your body weight is more of a factor in how much kite power you can handle than your strength. You should be strong enough to unhook the kite from your harness when you need to, though (do a lot of pull up). Kitesurfing is not very aerobic - you don't quickly run out of breath like you do when running. The kite does most of the work. Muscle fatigue can wear you out, but as your skills improve it becomes less strenuous.

I am a windsurfer, is it hard to convert?
As a windsurfer, you already know how to have good balance on a board and know the "way of the wind". It should be easier for a windsurfer to learn kitesurfing than for an ordinary person. However, the learning curve is still pretty steep as you need more balancing act in kitesurfing not to mention doing that while controlling a nervous kite which tends to pull you out of your board. Once you get pass the beginner stage, you can progress faster in kitesurfing than in windsurfing.

I am convinced now so how do I start?
It is wise to take lesson at a local reputable school.

Cost is an important consideration but not THE most important
consideration. Proper effective instruction can save you a lot of time,
frustration and possibly injury/damaged equipment in learning kiteboarding. Quality instruction is a good thing. If you are interested in kiteboarding take the intelligent path and hookup with a Professional kiteboarding instructor for adequate, quality training. The experience should speed you on your way to safer enjoyment of this excellent sport.