I want to use a checkbox to enable/disable an expression using keyframes. How can I do this?

Split the layer into respective sections with and without the expressions. No need to make it more complicated than that, especially since you have no expressions knowledge.

Mylenium

This is a keyframe overshoot expression from my site, which I've modified to skip the overshoot unless the checkbox is on at that keyframe. It may not exactly fit whatever you're using for overshoot, but hopefully it will give you an idea on how to incorporate the checkbox.

cb = effect("Checkbox Control")("Checkbox");

freq = 3;
decay = 5;

n = 0;
if (numKeys > 0){
  n = nearestKey(time).index;
  if (key(n).time > time) n--;
}
if (n > 0 && cb.valueAtTime(key(n).time)){
  t = time - key(n).time;
  amp = velocityAtTime(key(n).time - .001);
  w = freq*Math.PI*2;
  value + amp*(Math.sin(t*w)/Math.exp(decay*t)/w);
}else
  value