velocity = space/time. You look into your clock how much time the body or you take to cover s meters and you divide them: if you take 20 seconds to cover 500 meters, then your average velocity is 500/20 = 25 meters/seconds = 25 m/s. If you want it in Km/hours, multiply by 3.6: 25*3.6 = 90 Km/h.

This is called "average" velocity because, during those 20 seconds you could have accelerated to more than 90 Km/h or decelerated to less than it. The shorter the time interval, the more you are confident your velocity don't varies. At the limit, for a time interval extremely small, you can say that is your "istantaneous" velocity.

Once you have the velocity v, knowing the mass m, you can compute the kinetic energy E: E(kinetic) = m*v*v/2 = m*v^2/2.

For nitpicking people: that formula for kinetic energy is not valid any longer in relativity (that is, at very high speeds); it's not valid, e.g., for particles in a particle accelerator.